Irrigation, therefore, diminishes great rivers in warm countries by cutting off their sources of supply as well as by direct abstraction of water from their main channels. We have just seen that the system of irrigation in Lombardy deprives the Po of a quant.i.ty of water equal to the total delivery of the Seine at ordinary flood, or, in other words, of the equivalent of a tributary navigable for hundreds of miles by vessels of considerable burden.

The new ca.n.a.ls executed and projected will greatly increase the loss.

The water required for irrigation in Egypt is less than would be supposed from the exceeding rapidity of evaporation in that arid climate; for the soil is thoroughly saturated during the inundation, and infiltration from the Nile continues to supply a considerable amount of humidity in the dryest season. Linant Bey computed that, in the Delta, fifteen and one-third cubic yards per day sufficed to irrigate an acre.

If we suppose water to be applied for one hundred and fifty days during the season of growth, this would be equivalent to a total precipitation of about seventeen inches and one-third. Taking the area of actually cultivated soil in Egypt at the estimate of 4,500,000 acres, and the average amount of water daily applied in both Upper and Lower Egypt at twelve hundredths of an inch in depth, we have an abstraction of about 74,000,000 cubic yards, which--the mean daily delivery of the Nile being in round numbers 320,000,000 cubic yards--is twenty-three per cent of the average quant.i.ty of water contributed to the Mediterranean by that river. [Footnote: The proportion of the waters of the Nile withdrawn for irrigation is greater than this calculation makes it. The quant.i.ty required for an acre is less in the Delta than in Upper Egypt, both because the soil of the Delta, to which Linant Bey's estimate applies, lies little higher than the surface of the river, and is partly saturated by infiltration, and because near the sea, in N. L. 30 degrees, evaporation is much less rapid than it is several degrees southwards and in the vicinity of a parched desert.]

In estimating the effect of this abstraction of water upon the volume of great rivers, especially in temperate climates and in countries with a hilly surface, we must remember that all the water thus withdrawn--except that which is absorbed by vegetation, that which enters into new inorganic compounds, and that which is carried off by evaporation--is finally restored to the original current by superficial flow or by infiltration. It is generally estimated that from one-third to one-half of the water applied to the fields is absorbed by the earth, and this, with the deductions just given, is returned to the river by direct infiltration, or descends through invisible channels to moisten lower grounds, and thence in part escapes again into the bed of the river, by similar conduits, or in the form of springs and rivulets.

Interesting observations have lately been made on this subject in France and important practical results arrived at. It was maintained that mountain irrigation is not ultimately injurious to that of the plains below, because lands liberally watered in the spring, when the supply is abundant, act as reservoirs, storing up by absorption water which afterwards filters down to lower grounds or escapes into the channel of the river and keeps up its current in the dry summer months, so as to compensate for what, during those months, is withdrawn from it for upland irrigation. Careful investigation showed that though this proposition is not universally true, it is so in many cases, and there can be no doubt that the loss in the volume of rivers by the abstraction of water for irrigation is very considerably less than the measure of the quant.i.ty withdrawn. [Footnote: See Vigan, Etude sur les Irrigations, Paris, 1867; and Scott Moncrieff, Irrigation in Southern Europe, pp. 89, 90.

The brook Ain Musa, which runs through the ruined city of Petra and finally disappears in the sands of Wadi el Araba, is a considerable stream in winter, and the inhabitants of that town were obliged to excavate a tunnel through the rock near the right bank, just above the upper entrance of the narrow Sik, to discharge a part of its swollen current. The sagacity of Dr. Robinson detected the necessity of this measure, though the tunnel, the mouth of which was hidden by brushwood, was not discovered till some time after his visit. I even noticed, near the arch that crosses the Sik, unequivocal remains of a sluice by which the water was diverted to the tunnel. Immense labor was also expended in widening the natural channel at several points below the town, to prevent the damming up and setting back of the water--a fact I believe not hitherto noticed by travellers.

The Fellahheen above Petra still employ the waters of Ain Musa for irrigation, and in summer the superficial current is wholly diverted from its natural channel for that purpose. At this season, the bed of the brook, which is composed of pebbles, gravel, and sand, is dry in the Sik and through the town; but the infiltration is such that water is generally found by digging to a small depth in the channel. Observing these facts in a visit to Petra in the summer, I was curious to know whether the subterranean waters escaped again to daylight, and I followed the ravine below the town for a long distance. Not very far from the upper entrance of the ravine, arborescent vegetation appeared upon its bottom, and as soon as the ground was well shaded, a thread of water burst out. This was joined by others a little lower down, and, at the distance of a mile from the town, a strong current was formed and ran down towards Wadi el Araba.

Similar facts are observed in all countries where the superficial current of water-courses is diverted from their bed for irrigation, but this case is of special interest because it shows the extent of absorption and infiltration even in the torrid climate of Arabia. See Baird Smith, Italian Irrigation, vol. i., pp. 172, 386 and 387.]

Irrigation, as employed for certain special purposes in Europe and America, is productive of very prejudicial climatic effects. I refer particularly to the cultivation of rice in the Southern States of the American Union and in Italy. The climate of the Southern States is in general not necessarily unhealthy for the white man, but he can scarcely sleep a single night in the vicinity of the rice-grounds without being attacked by a dangerous fever. The neighborhood of the rice-fields is possibly less pestilential in Lombardy and Piedmont than in South Carolina and Georgia, but still very insalubrious to both man and beast.

"Not only does the population decrease where rice is grown," says Escourron-Milliago, "but even the flocks are attacked by typhus. In the rice-grounds the soil is divided into compartments rising in gradual succession to the level of the irrigating ca.n.a.l, in order that the water, after having flowed one field, may be drawn off to another, and thus a single current serve for several compartments, the lowest field, of course, still being higher than the ditch which at last drains both it and the adjacent soil. This arrangement gives a certain force of hydrostatic pressure to the water with which the rice is irrigated, and the infiltration from these fields is said to extend through neighboring grounds, sometimes to the distance of not less than a myriametre, or six English miles, and to be destructive to crops and even trees reached by it. Land thus affected can no longer be employed for any purpose but growing rice, and when prepared for that crop, it propagates still further the evils under which it had itself suffered, and, of course, the mischief is a growing one." [Footnote: Escourrou-Milliago, D'Italie a propos de l'Exposition de Paris, 1856, p. 92. According to an article in the Gazzetto di Torino for the 17th of January, 1869, the deaths from malarious fever in the Canavese district--which is a.s.serted to have been altogether free from this disease before the recent introduction of rice-culture--between the 1st of January and the 15th of October, 1868, were two thousand two hundred and fifty. The extent of the injurious influence of this very lucrative branch of rural industry in Italy is contested by the rice-growers. But see Secondo Laura, Le Risaje, Torino, 1869; Selmi, Il Miasma Pal.u.s.tre, p. 89; and especially Carlo Livi, Della coltivazione del Riso in Italia, in the Nuova Antologia for July, 1871, p. 599 et seqq. According to official statistics, the rice-grounds of Italy, including the islands, amounted in 1866 to 450,000 acres. It is an interesting fact in relation to geographical and climatic conditions, that while little rice is cultivated SOUTH of N. L. 44 degrees in Italy, little is grown in the United States NORTH of 35 degrees. To the southward of the great alluvial plain of the Po, the surface is in general too much broken to admit of the formation of level fields of much extent, and where the ground is suitable, the supply of water is often insufficient.

The Moors introduced the cultivation of rice into Spain at an early period of their dominion in that country. The Spaniards sowed rice in Lombardy and in the Neapolitan territory in the 16th century; but besides the want of water and of level ground convenient for irrigation, rice-husbandry has proved so much more pestilential in Southern than in Northern Italy that it has long been discouraged by the Neapolitan government.]

Salts deposited by Water of Irrigation.

The attentive traveller in Egypt and Nubia cannot fail to notice many localities, generally of small extent, where the soil is rendered infertile by an excess of saline matter in its composition. In many cases, perhaps in all, these barren spots lie rather above the level usually flooded by the inundations of the Nile, and yet they exhibit traces of former cultivation. Observations in India suggest a possible explanation of this fact. A saline efflorescence called "Reh" and "Kuller" is gradually invading many of the most fertile districts of Northern and Western India, and changing them into sterile deserts. It consists princ.i.p.ally of sulphate of soda (Glauber's salts), with varying proportions of common salt. These salts (which in small quant.i.ties are favorable to fertility of soil) are said to be the gradual result of concentration by evaporation of river and ca.n.a.l waters, which contain them in very minute quant.i.ties, and with which the lands are either irrigated or occasionally overflowed. The river inundations in hot countries usually take place but once in a year, and, though the banks remain submerged for days or even weeks, the water at that period, being derived princ.i.p.ally from rains and snows, must be less highly charged with mineral matter than at lower stages, and besides, it is always in motion. The water of irrigation, on the other hand, is applied for many months in succession, it is drawn from rivers and ca.n.a.ls at the seasons when the proportion of salts is greatest, and it either sinks into the superficial soil, carrying with it the saline substances it holds in solution, or is evaporated from the surface, leaving them upon it. Hence irrigation must impart to the soil more salts than natural inundation.

The sterilized grounds in Egypt and Nubia lying above the reach of the floods, as I have said, we may suppose them to have been first cultivated in that remote antiquity when the Nile valley received its earliest inhabitants, and when its lower grounds were in the condition of mora.s.ses. They must have been artificially irrigated from the beginning; they may have been under cultivation many centuries before the soil at a lower level was invaded by man, and hence it is natural that they should be more strongly impregnated with saline matter than fields which are exposed every year, for some weeks, to the action of running water so nearly pure that it would be more likely to dissolve salts than to deposit them.

SUBTERRANEAN WATERS.

I have frequently alluded to a branch of physical geography, the importance of which is but recently adequately recognized--the subterranean waters of the earth considered as stationary reservoirs, as flowing currents, and as filtrating fluids. The earth drinks in moisture by direct absorption from the atmosphere, by the deposition of dew, by rain and snow, by percolation from rivers and other superficial bodies of water, and sometimes by currents flowing into caves or smaller visible apertures. [Footnote: The great limestone plateau of the Karst in Carniola is completely honey-combed by caves through which the drainage of that region is conducted. Rivers of considerable volume pour into some of these caves and can be traced underground to their exit.

Thus the Recca has been satisfactorily identified with a stream flowing through the cave of Trebich, and with the Timavo--the Timavus of Virgil and the ancient geographers--which empties through several mouths into the Adriatic between Trieste and Aquileia. The city of Trieste is very insufficiently supplied with fresh water. It has been thought practicable to supply this want by tunnelling through the wall of the plateau, which rises abruptly in the rear of the town, until some subterranean stream is encountered, the current of which can be conducted to the city. More visionary projectors have gone further, and imagined that advantage might be taken of the natural tunnels under the Karst for the pa.s.sage of roads, railways, and even navigable ca.n.a.ls. But however chimerical these latter schemes may seem, there is every reason to believe that art might avail itself of these galleries for improving the imperfect drainage of the champaign country bounded by the Karst, and that stopping or opening the natural channels might very much modify the hydrography of an extensive region. See in Aus des Natur, xx., pp.

250-254, 263-266, two interesting articles founded on the researches of Schmidt.

The cases are certainly not numerous where marine currents are known to pour continuously into cavities beneath the surface of the earth, but there is at least one well-authenticated instance of this sort--that of the mill-stream at Argostoli in the island of Cephalonia. It had been long observed that the sea-water flowed into several rifts and cavities in the limestone rocks of the coast, but the phenomenon has excited little attention until very recently. In 1833, three of the entrances were closed, and a regular channel, sixteen feet long and three feet wide, with a fall of three feet, was cut into the mouth of a larger cavity. The sea-water flowed into this ca.n.a.l, and could be followed eighteen or twenty feet beyond its inner terminus, when it disappeared in holes and clefts in the rock.

In 1858 the ca.n.a.l had been enlarged to thewidth of five feet and a half, and a depth of a foot. The water pours rapidly through the ca.n.a.l into an irregular depression and forms a pool, the surface of which is three or four feet below the adjacent soil, and about two and a half or three feet below the level of the sea. From this pool it escapes through several holes and clefts in the rock, and has not yet been found to emerge elsewhere.

There is a tide at Argostoli of about six inches in still weather, but it is considerably higher with a south wind. I do not find it stated whether water flows through the ca.n.a.l into the cavity at low tide, but it distinctly appears that there is no refluent current, as of course there could not be from a base so much below the sea. Mousson found the delivery through the ca.n.a.l to be at the rate of 24.88 cubic feet to the second; at what stage of the tide does not appear. Other mills of the same sort have been erected, and there appear to be several points on the coast where the sea flows into the land.

Various hypotheses have been suggested to explain this phenomenon, some of which a.s.sume that the water descends to a great depth beneath the crust of the earth; but the supposition of a difference of level in the surface of the sea on the opposite sides of the island, which seems confirmed by other circ.u.mstances, is the most obvious method of explaining these singular facts. If we suppose the level of the water on one side of the island to be raised by the action of currents three or four feet higher than on the other, the existence of cavities and channels in the rock would easily account for a subterranean current beneath the island, and the apertures of escape might be so deep or so small as to elude observation. See Aus der Natur, vol. xix., pp. 129 et seqq. I have lately been informed by a resident of the Ionian Islands, who is familiar with the locality, that the sea flows uninterruptedly into the sub-insular cavities, at all stages of the tide.] Some of this humidity is exhaled again by the soil, some is taken up by organic growths and by inorganic compounds, some poured out upon the surface by springs and either immediately evaporated or carried down to larger streams and to the sea, some flows by subterranean courses into the bed of fresh-water rivers [Footnote: "The affluents received by the Seine below Rouen are so inconsiderable, that the augmentation of the volume of that river must be ascribed princ.i.p.ally to springs rising in its bed.

This is a point of which engineers now take notice, and M. Belgrand, the able officer charged with the improvement of the navigation of the Seine between Paris and Rouen, has devoted much attention to it."--Babinet, Etudes et Lectures, iii., p. 185.

On page 232 of the volume just quoted, the same author observes: "In the lower part of its course, from the falls of the Oise, the Seine receives so few important affluents, that evaporation alone would suffice to exhaust all the water which pa.s.ses under the bridges of Paris."

This supposes a much greater amount of evaporation than has been usually computed, but I believe it is well settled that the Seine conveys to the sea much more water than is discharged into it by all its superficial branches. Babinet states the evaporation from the surface of water at Paris to be twice as great as the precipitation.

Belgrand supposes that the floods of the Seine at Paris are not produced by the superficial flow of the water of precipitation into its channel, but from the augmented discharge of its remote mountain sources, when swollen by the rains and melted snows which percolate through the permeable strata in its upper course.--Annales des Ponts et Chaussees, 1851, vol. i.] or of the ocean, and some remains, though even here not in forever motionless repose, to fill deep cavities and underground channels. In every case the aqueous vapors of the air are the ultimate source of supply and all these hidden stores are again returned to the atmosphere by evaporation.

The proportion of the water of precipitation taken up by direct evaporation from the surface of the ground seems to have been generally exaggerated, sufficient allowance not being made for moisture carried downwards or in a lateral direction by infiltration or by crevices in the superior rocky or earthy strata. According to Wittwer, Mariotte found that but one-sixth of the precipitation in the basin of the Seine was delivered into that sea by the river, "so that five-sixths remained for evaporation and consumption by the organic world." [Footnote: Physicalische Geographie, p. 286. It does not appear whether this inference is Mariotte's or Wittwer's. I suppose it is a conclusion of the latter.

According to Valles, the Seine discharges into the sea thirty per cent.

of the precipitation in its valley, while the Po delivers into the Adriatic two-thirds and perhaps even three-quarters of the total down-fall of its basin. The differences between the tributaries of the Mississippi in this respect are remarkable, the Missouri discharging only fifteen per cent., the Yazoo not less than ninety. The explanation of these facts is found in the geographical and geological character of the valleys of these rivers. The Missouri flows with a rapid current through an irregular country, the Yazoo has a very slow flow through a low, alluvial region which is kept constantly almost saturated by infiltration.] Maury estimates the annual amount of precipitation in the valley of the Mississippi at 620 cubic miles, the discharge of that river into the sea at 107 cubic miles, and concludes that "this would leave 513 cubic miles of water to be evaporated from this river-basin annually." [Footnote: Physical Geography of the Sea. Tenth edition.

London, 1861, Section 274.] In these and other like computations, the water carried down into the earth by capillary and larger conduits is wholly lost sight of, and no thought is bestowed upon the supply for springs, for common and artesian wells, and for underground rivers, like those in the great caves of Kentucky, which may gush up in fresh-water currents at the bottom of the Caribbean Sea, or rise to the light of day in the far-off peninsula of Florida. [Footnote: In the low peninsula of Florida, rivers, which must have their sources in mountains hundreds of miles distant, pour forth from the earth with a volume sufficient to permit steamboats to ascend to their basins of eruption. In January, 1857, a submarine fresh-water river burst from the bottom of the sea not far from the southern extremity of the peninsula, and for a whole month discharged a current not inferior in volume to the river Mississippi, or eleven times the mean delivery of the Po, and more than six times that of the Nile. We can explain this phenomenon only by supposing that the bed of the sea was suddenly burst up by the hydrostatic upward pressure of the water in a deep reservoir communicating with some great subterranean river or receptacle in the mountains of Georgia or of Cuba, or perhaps even in the valley of the Mississippi.--Thoma.s.sy, Essai sur l'Hydrologie. Late southern journals inform us that the creek under the Natural Bridge in Virginia has suddenly disappeared, being swallowed up by newly formed fissures, of unknown depth, in its channel. It does not appear that an outlet for the waters thus absorbed has been discovered, and it is not improbable that they are filling some underground cavity like that which supplied the submarine river just mentioned.]

The progress of the emphatically modern science of geology has corrected these erroneous views, because the observations on which it depends have demonstrated not only the existence, but the movement, of water in nearly all geological formations, have collected evidence of the presence of large reservoirs at greater or less depths beneath surfaces of almost every character, and have investigated the rationale of the attendant phenomena. [Footnote: See especially Stoppani, Corso di Geologia, i., pp. 270 et seqq.] The distribution of these waters has been minutely studied with reference to a great number of localities, and though the actual mode and rate of their vertical and horizontal transmission is still involved in much obscurity, the laws which determine their aggregation are so well understood, that, when the geology of a given district is known, it is not difficult to determine at what depth water will be reached by the borer, and to what height it will rise. The same principles have been successfully applied to the discovery of small subterranean collections or currents of water, and some persons have acquired, by a moderate knowledge of the superficial structure of the earth combined with long practice, a skill in the selection of favorable places for digging wells which seems to common observers little less than miraculous. The Abbe Paramelle--a French ecclesiastic who devoted himself for some years to this subject and was extensively employed as a well-finder--states, in his work on Fountains, that in the course of thirty-four years he had pointed out more than ten thousand subterranean springs; and though his geological speculations were often erroneous, high scientific authorities have testified to the great practical value of his methods, and the general accuracy of his predictions. [Footnote: Paramelle, Quellenkunde, mit einem Vorwort von B. Cotta. 1856.] Hydrographical researches have demonstrated the existence of subterranean currents and reservoirs in many regions where superficial geology had not indicated their probable presence. Thus, a much larger proportion of the precipitation in the valley of the Tiber suddenly disappears than can be accounted for by evaporation and visible flow into the channel of the river. Castelli suspected that the excess was received by underground caverns, and slowly conducted by percolation to the bed of the Tiber. Lombardini--than whom there is no higher authority--concludes that the quant.i.ty of water gradually discharged into the river by subterranean conduits, is not less than three-quarters of the total delivery of its basin. [Footnote: See Lombardini, Importanza degli studi sulla Statistica da Fiumi, p. 27; also, same author, Sulle Inondazioni avvenute in Francia, etc., p. 29.] What is true of the hydrology of the Tiber is doubtless more or less true of that of other rivers, and the immense value of natural arrangements which diminish the danger of sudden floods by retaining a large proportion of the precipitation, and of an excessive reduction of river currents in the droughts of summer, by slowly conducting into their beds water acc.u.mulated and stored up in subterranean reservoirs in rainy seasons, is too obvious to require to be dwelt upon. The readiness with which water not obstructed by impermeable strata diffuses itself through the earth in all directions--and consequently, the importance of keeping up the supply of subterranaean reservoirs--find a familiar ill.u.s.tration in the effect of paving the ground about the stems of vines and trees.

The surface-earth around the trunk of a tree may be made almost impervious to water, by flagstones and cement, for a distance as great as the spread of the roots; and yet the tree will not suffer for want of moisture, except in droughts severe enough sensibly to affect the supply in deep wells and springs. Both forest and fruit trees attain a considerable age and size in cities where the streets and courts are closely paved, and where even the lateral access of water to the roots is more or less obstructed by deep cellars and foundation walls. The deep-lying veins and sheets of water, supplied by infiltration from often comparatively distant sources, send up moisture by capillary attraction, and the pavement prevents the soil beneath it from losing its humidity by evaporation. Hence, city-grown trees find moisture enough for their roots, and though plagued with smoke and dust, often retain their freshness, while those planted in the open fields, where sun and wind dry up the soil faster than the subterranean fountains can water it, are withering from drought. [Footnote: The roots of trees planted in towns do not depend exclusively on infiltration for their supply of water, for they receive a certain amount of both moisture and air through the interstices between the paving-stones; but where wide surfaces of streets and courts are paved with air and water tight asphaltum, as in Paris, trees suffer from the diminished supply of these necessary elements.] Without the help of artificial conduit or of water-carrier, the Thames and the Seine refresh the ornamental trees that shade the thoroughfares of London and of Paris, and beneath the hot and reeking mould of Egypt, the Nile sends currents to the extremest border of its valley. [Footnote: See the interesting observations of Krieck on this subject, Schriften zur allgemeinen Erdkunde, cap. iii., Section 6, and especially the pa.s.sages in Ritter s Erdkunde, vol. i., there referred to.

The tenacity with which the parched soil of Egypt retains the supply of moisture it receives from the Nile is well ill.u.s.trated by observations of Girard cited by Lombardini from the Memoires de l'Academie des Sciences, t. ii., 1817. Girard dug wells at distances of 3,200, 1,800, and 1,200 metres from the Nile, and after three months of low water in the river, found water in the most remote well, at 4m. 97, in the next at 4m. 23, and in that nearest the bank at 3m. 44 above the surface of the Nile. The fact that the water was highest in the most distant well appears to show that it was derived from the inundation and not, by lateral infiltration, from the river. But water is found beneath the sands at points far above and beyond the reach of the inundations, and can be accounted for only by subterranean percolation from the Nile. At high flood, the hydrostatic pressure on the banks, combined with capillary attribution, sends water to great horizontal distances through the loose soil; at low water the current is reversed, and the moisture received from the river is partly returned, and may often be seen oozing from the banks into the river.--Clot Bey, Apercu sur l'Egypte, i., 128.

Laurent (Memoires sur le Sahara Oriental, pp. 8, 9), in speaking of a river at El-Faid, "which, like all those of the desert, is, most of the time, without water," observes, that many wells are dug in the bed of the river in the dry season, and that the subterranean supply of water thus reached extends itself laterally, at about the same level, at least a kilometre from the river, as water is found by digging to the depth of twelve or fifteen metres at a village situated at that distance from the bank.

Recent experiments, however, have shown that in the case of rivers flowing through thickly peopled regions, and especially where the refuse from industrial establishments is discharged into them, the finely comminuted material received from sewers and factories sometimes clogs up the interstices between the particles of sand and gravel which compose the bed and banks, and the water is consequently confined to the channel and no longer diffuses itself laterally through the adjacent soil. This obstruction of course acts in both directions, according to circ.u.mstances. In one case, it prevents the escape of river-water and tends to maintain a full flow of the current; in another it intercepts the supply the river might otherwise receive by infiltration from the land, and thus tends to reduce the volume of the stream. In some instances pits have been sunk along the banks of large rivers and the water which filters into them pumped up to supply aqueducts. This method often succeeds, but where the bed of the stream has been rendered impervious by the discharge of impurities into it, it cannot be depended upon.

The tubular wells generally known as the American wells furnish another proof of the free diffusion and circulation of water through the soil. I do not know the date of the first employment of these tubes in the United States, but as early as 1861, the Chevalier Calandra used wooden tubes for this pose in Piedmont, with complete success. See the interesting pamphlet, Sulla Estrazione delle Acque Sotterrance, by C.

Calandra. Torino, 1867.

The most remarkable case of infiltration known to me by personal observation is the occurrence of fresh water in the beach-sand on the eastern side of the Gulf of Akaba, the eastern arm of the Red Sea. If you dig a cavity in the beach near the sea-level, it soon fills with water so fresh as not to be undrinkable, though the sea-water two or three yards from it contains even more than the average quant.i.ty of salt. It cannot be maintained that this is sea-water freshed by filtration through a few feet or inches of sand, for salt-water cannot be deprived of its salt by that process. It can only come from the highlands of Arabia, and it would seem that there must exist some large reservoir in the interior to furnish a supply which, in spite of evaporation, holds out for months after the last rains of winter, and perhaps even through the year. I observed the fact in the month of June.

See Robinson, Biblical Researches, 1857, vol i., p. 167.

The precipitation in the mountains that border the Red Sea is not known by pluviometric measurement, but the ma.s.s of debris brought down the ravines by the torrents proves that their volume must be large. The proportion of surface covered by sand and absorbent earth, in Arabia Petraea and the neighboring countries, is small, and the mountains drain themselves rapidly into the wadies or ravines where the torrents are formed; but the beds of earth and disintegrated rock at the bottom of the valleys are of so loose and porous texture, that a great quant.i.ty of water is absorbed in saturating them before a visible current is formed on their surface. In a heavy thunder-storm, accompanied by a deluging rain, which I witnessed at Mount Sinai in the month of May, a large stream of water poured, in an almost continuous cascade, down the steep ravine north of the convent, by which travellers sometimes descend from the plateau between the two peaks, but after reaching the foot of the mountain, it flowed but a few yards before it was swallowed up in the sands.

Fresh-water wells are not unfrequently found upon the borders of ocean beaches. In the dry summer of 1870, drinkable water was procured in many places on the coast of Liguria by digging to the depth of a yard in the beach-sands. Tubular wells reach fresh water at twelve or fifteen feet below the surface on the sandy plains of Cape Cod. In this latter case, the supply is more probably derived directly from precipitation than from lateral infiltration.]

Artesian Wells.

The existence of artesian wells depends upon that of subterranean reservoirs and rivers, and the supply yielded by borings is regulated by the abundance of such sources. The waters of the earth are, in many cases, derived from superficial currents which are seen to pour into chasms opened, as it were, expressly for their reception; and in others, where no apertures in the crust of the earth have been detected, their existence is proved by the fact that artesian wells sometimes bring up from great depths seeds, leaves, and even living fish, which must have been carried down through channels large enough to admit a considerable stream. [Footnote: Charles Martins, Le Sahara, in Revue des Deux Mondes, Sept. 1, 1864, p. 619; Stoppani, Corso di Geologia, i., 281; Desor, Die Sahara, Basel, 1871, pp. 50, 51.] But in general, the sheet and currents of water reached by deep boring appear to be primarily due to infiltration from highlands where the water is first collected in superficial or subterranean reservoirs. By means of channels conforming to the dip of the strata, these reservoirs communicate with the lower basins, and exert upon them a fluid pressure sufficient to raise a column to the surface, whenever an orifice is opened. [Footnote: It is conceivable that in shallow subterranean basins superinc.u.mbered mineral strata may rest upon the water and be partly supported by it. In such case the weight of such strata would be an additional, if not the sole, cause of the ascent of the water through the tubes of artesian wells.

The ascent of petroleum in the artesian oil-wells in Pennsylvania, and, in many cases, of salt-water in similar tubes, can hardly be ascribed to hydrostatic pressure, and there is much difficulty in accounting for the rise of water in artesian wells in many parts of the African desert on that principle. Perhaps the elasticity of gases, which probably aids in forcing up petroleum and saline waters, may be, not unfrequently, an agency in causing the flow of water in common artesian borings. It is said that artesian wells lately bored in Chicago, some to the depth of 1,600 feet, raise water to the height of 100 feet above the surface.

What is the source of the pressure ] The water delivered by an artesian well is, therefore, often derived from distant sources, and may be wholly unaffected by geographical or meteorological changes in its immediate neighborhood, while the same changes may quite dry up common wells and springs which are fed only by the local infiltration of their own narrow basins.

In most cases, artesian wells have been bored for purely economical or industrial purposes, such as to obtain good water for domestic use or for driving light machinery, to reach saline or other mineral springs, and recently, in America, to open fountains of petroleum or rock-oil.

The geographical and geological effects of such abstraction of fluids from the bowels of the earth are too remote and uncertain to be here noticed; [Footnote: Many more or less probable conjectures have been made on this subject but thus far I am not aware that any of the apprehended results have been actually shown to have happened. In an article in the Annales des Ponts et Chaussees for July and August, 1839, p. 131, it was suggested that the sinking of the piers of a bridge at Tours in France was occasioned by the abstraction of water from the earth by artesian wells, and the consequent withdrawal of the mechanical support it had previously given to the strata containing it. A reply to this article will be found in Viollet, Theorie des Puits Artesiens, p.

217.

In some instances the water has rushed up with a force which seemed to threaten the inundation of the neighborhood, and even the washing away of much soil; but in those cases the partial exhaustion of the supply, or the relief of hydrostatic or elastic pressure, has generally produced a diminution of theflow in a short time, and I do not know that any serious evil has everbeen occasioned in this way.

In April, 1866, a case of this sort occurred in boring an artesian well near the church of St. Agnes at Venice. When the drill reached the depth of 160 feet, a jet of mud and water was shot up to the height of 130 feet above the surface, and continued to flow with gradually diminishing force for about eight hours.] but artesian wells have lately been employed in Algeria for a purpose which has even now a substantial, and may hereafter acquire a very great geographical importance. It was observed by many earlier as well as recent travellers in the East, among whom Shaw deserves special mention, that the Libyan desert, bordering upon the cultivated sh.o.r.es of the Mediterranean, appeared in many places to rest upon a subterranean lake at an accessible distance below the surface. The Moors are vaguely said to bore artesian wells down to this reservoir, to obtain water for domestic use and irrigation, and there is evidence that this art was practised in Northern Africa in the Middle Ages. But it had been lost by the modern Moors, and the universal astonishment and incredulity with which the native tribes viewed the operations of the French engineers sent into the desert for that purpose, are a sufficient proof that this mode of reaching the subterranean waters was new to them. They were, however, aware of the existence of water below the sands, and were dexterous in digging wells--square shafts lined with a framework of palm-tree stems--to the level of the sheet. The wells so constructed, though not technically artesian wells, answer the same purpose; for the water rises to the surface and flows over it as from a spring. [Footnote: See a very interesting account of these wells, and of the workmen who clean them out when obstructed by sand brought up with the water, in Laurent's memoir on the artesian wells recently bored by the French Government in the Algerian desert. Mimoire sur le Sahara Oriental, etc., pp. et seqq.

Some of the men remained under water from two minutes to two minutes and forty seconds. Several officers are quoted as having observed immersions of three minutes' duration, and M. Berbrugger witnessed ona of six minutes and five seconds and another of five minutes and fifty-five seconds. The shortest of these periods is longer than the best pearl-diver can remain below the surface of salt-water. The wells of the Sahara are from twenty to eighty metres deep.-Desor, Die Sahara, Basel, 1871, p. 43.

The ancient Egyptians were acquainted with the art of boring artesian wells. Ayme, a French engineer in the service of the Pacha of Egypt, found several of these old wells, a few years ago, in the oases. They differed little from modern artesian wells, but were provided with pear-shaped valves of stone for closing them when water was not needed.

When freed from the sand and rubbish with which they were choked, they flowed freely and threw up fish large enough for the table. The fish were not blind, as cave-fish often are, but were provided with eyes, and belonged to species common in the Nile. The sand, too, brought up with them resembled that of the bed of that river. Hence it is probable that they were carried to the oases by subterranean channels from the Nile.--Desor, Die Sahara, Basel, 1871, p. 28; Stoppani, Corso di Geologia, i., p. 281. Barth speaks of common wells in Northern Africa from 200 to 360 feet deep.--Reisen in Africa, ii., p. 180.

It is certain that artesian wells have been common in China from a very remote antiquity, and the simple method used by the Chinese--where the drill is raised and let fall by a rope, instead of a rigid rod--has lately been employed in Europe with advantage. Some of the Chinese wells are said to be 3,000 feet deep; that of Neusalzwerk in Silesia is 2,300.

A well was bored at St. Louis, in Missouri, a few years ago, to supply a sugar refinery, to the depth of 2,199 feet. This was executed by a private firm in three years, at the expense of only $10,000. Four years since the boring was recommenced in this well and reached a depth of 3,150 feet, but without a satisfactory result. Another artesian well was sunk at Columbus, in Ohio, to the depth of 2,500 feet, but without obtaining the desired supply of water. Perhaps, however, the artesian well of the greatest depth ever executed until very recently, is that bored within the last six or seven years, for the use of an Insane Asylum near St. Louis. This well descends to the depth of three thousand eight hundred and forty-three feet, but the water which it furnishes is small in quant.i.ty and of a quality that cannot be used for ordinary domestic purposes. The bore has a diameter of six inches to the depth of 425 feet, and after that it is reduced to four inches. For about three thousand feet the strata penetrated were of carboniferous and magnesian limestone alternating with sandstone. The remainder of the well pa.s.ses through igneous rock. At St. Louis the Missouri and Mississippi rivers are not more than twenty miles distant from each other, and it is worthy of note that the waters of neither of those two rivers appear to have opened for themselves a considerable subterranean pa.s.sage through the rocky strata of the peninsula which separates them.

When in boring an artesian well water is not reached at a moderate depth, it is not always certain that it will be found by driving the drill still lower. In certain formations, water diminshes as we descend, and it seems probable that, except in case of caverns and deep fissures, the weight of the superinc.u.mbent mineral strata so compresses the underlying ones, at no very great distance below the surface, as to render them impermeable to water and consequently altogether dry. See London Quarterly Journal of Science, No. xvii., Jan., 1868, p. 18, 19.

In the silver mines of Nevada water is scarcely found at depths below 1,000 feet, and at 1,200 feet from the surface the earth is quite dry.--American Annual of Scientific Discovery for 1870, p. 75.

Similar facts are observed in Australia. The Pleasant Creek News writes: "A singular and unaccountable feature in connection with our deep quartz mines is being developed daily, which must surprise those well experienced in mining matters. It is the decrease of water as the greater depths are reached. In the Magdala shaft at 950 ft. the water has decreased to a MINIMUM; in the Crown Cross Reef Company's shaft, at 800 ft., notwithstanding the two reefs recently struck, no extra water has been met with; and in the long drive of the Extended Cross Reef Company, at a depth of over 800 ft., the water is lighter than it was nearer the surface."