[Ill.u.s.tration: Fig. 84.]

=111. Glaisher's Rain-Gauge.=--The rain-gauge designed by Mr. Glaisher, the well-known meteorologist, and used by most observers of the present day, is arranged for the reception of the water which falls upon its receiving surface only, and for the prevention of loss by evaporation. The rain is first collected in a funnel, _B_, (fig. 84,) the receiving surface of which is turned in a lathe. The conical surface of the funnel slopes to the pipe, _E_, at an angle of 60 from the horizontal receiving surface.

The tube, _E_, is of small aperture, and is bent up, in order to retain the last few drops of rain, so that the only opening for the escape of vapour may be closed as long as possible. The funnel, _B_, fits upon the cylinder, _A_, tightly in the groove, _D_. A copper can is placed inside the cylinder, _A_, to receive the rain from the funnel. Once or twice a day, or after a shower, this can should be taken out, and the water measured in the gla.s.s measure, _C_, which is graduated to hundredths of an inch, according to the calculated quant.i.ty of water, determined by the area of the receiving s.p.a.ce. In use, this gauge should be partly sunk in the ground, so that the top may be about five inches above it. Thus situated, there will be little or no evaporation from it during any month of the year; and the readings need not be taken daily, although desirable.

=112. Rain-Gauge with Float.=--In this construction the graduated gla.s.s measure is dispensed with. The cylinder of the gauge is made less in diameter than the funnel, and a hollow, very flattened spheroid of copper forming a float, and carrying a vertical graduated boxwood scale which moves through the orifice of the funnel, is placed in it. As the rain acc.u.mulates the float rises, and the amount of rain in the gauge is read upon the scale from the top of the gauge, a bar, having a hole at the centre for the pa.s.sage of the scale, being fixed diametrically across the receiving s.p.a.ce of the funnel. The gauge is provided at the bottom with a bra.s.s c.o.c.k, by which the water may be allowed to flow out of it whenever necessary.

This form of gauge is not very suitable for the measurement of small quant.i.ties; but is admirably adapted for localities where the rainfall is excessive.

[Ill.u.s.tration: Fig. 85.]

=113. Rain-Gauge with Side-Tube.=--This instrument, as represented in fig.

85, is a cylindrical vessel, mounted on a base shaped as a frustum of a cone. This base may be filled with sand or gravel to make the instrument stable, so that when placed upon a lawn or in a garden it may have an ornamental appearance. The funnel for collecting the rain is larger in diameter than the cylinder. Parallel to the cylinder, and communicating with the lowest part of the interior and extending to its top, is a graduated gla.s.s tube, open at both ends. The rain collected will rise as high in this tube as in the cylinder, and its amount can therefore be read off without any trouble. The gauge is emptied by the bra.s.s tap at the bottom of the cylinder.

=114. Admiral FitzRoy's Rain-Gauge.=--A form of rain-gauge, very well adapted for expeditious observation at any time, has been designed by Admiral FitzRoy, and extensively employed by his observers. It is cylindrical in shape, with the funnel let into the top; and the rainfall is collected in an inner and much smaller cylinder, so that a small fall is represented by a considerable depth of water in the gauge. The amount of rain which has fallen is ascertained by a dipping tube, similar in principle to the dipping syphon used by gaugers for taking out specimens of wines or spirits from casks by simply removing the bung. A short, vertical, tubular opening provided with a cap, which is attached to the instrument by a chain that it may not be lost, is formed in the funnel.

The measuring tube, which has a small hole at each end, should be placed upright in the gauge; then the thumb should be pressed over the upper aperture, while the tube is lifted gently out, holding in the lower part a quant.i.ty of water representing the depth of the rain in the gauge, the upper edge of which is at the mark to be read off. The gla.s.s tube is graduated to inches and tenths; hundredths of an inch can be readily estimated by the eye. The marks are fixed by actual trial with a standard gauge, and are artificial, not true, inches.

=115. Self-Registering Rain-Gauge.=--The rain-gauge can be combined with clock-work and other mechanism so as to be self-recording of the amount of rain, the time, and duration of its fall. For the details of construction the reader is referred to the next chapter, where he will find the instrument described in connection with Osler's anemometer, as the "pluviometer." To observe and duly record the times of commencement and termination of rain is very desirable. Scarcely any observer can attempt to do this even approximately from personal observation. Hence the want of a cheap and simple self-recording rain-gauge is much felt, the present construction being too expensive for all but a few individuals.

In 1862, Mr. R. Strachan estimated the duration and amount of rain in London (Gray's Inn Road) as follows:--

+-------------------------------------------------------------------+

MONTHS.

INCHES.

DAYS.

HOURS.

MONTHS.

INCHES.

DAYS.

HOURS.

-----------+-------+-----+------++------------+-------+-----+------

January.

186

19

88

July.

227

17

68

February.

037

9

25

August.

245

12

72

March.

340

22

130

September.

170

12

55

April.

234

14

80

October.

323

21

94

May.

304

16

90

November.

112

10

53

June.

245

20

83

December.

144

17

66

+-------------------------------------------------------------------+

"During the year 1862, the rainfall amounted to 2567 inches. Rain fell on 179 days, that is, on nearly every other day. The hours of rain were estimated at 904; therefore, if the rain had fallen continuously, it would have lasted nearly 38 days and nights."[10] The value of similar estimates of the rainfall by numerous observers would be very great to meteorology.

=116. The principle of measurement= in all these gauges is the relation existing between the areas of the collecting and receiving surfaces; that is, between the area of the funnel into which the rain falls, and the area of the cylinder which receives it. In Howard's and Glaisher's gauges, this cylinder is virtually the measuring gla.s.s itself; in the others, above described, the measuring scales show the same depth of water as in the cylinder of the gauge.

The cylinder being of less diameter than the funnel, and receiving all the rain collected by the funnel, it follows that its contents will have an increased depth. Now equal cylindrical volumes, having different diameters, are to each other in length inversely as the squares of the diameters. Hence, if the funnel be 9 inches and the cylinder 3 inches in diameter, a fall of 1 inch of rain will be represented in the gauge by 9 inches; for 3 : 9 :: 1 : _x_ = 9. In this case, therefore, a length of nine inches of the measuring gla.s.s, tube, or scale, would represent an inch of rainfall, and be divided into tenths and hundredths of the artificial inch.

=117. Position for Rain-Gauge, &c.=--Rain gauges should be placed on the ground, in any position exposed to a free fall of rain, snow, or hail, where neither walls, buildings, nor trees shelter or cause eddies of wind.

They should be supported by a frame, or other means, to prevent them being blown down by the wind, but so that they can be readily emptied.

During snow or frost, the gauge must be watched, and its contents melted by placing it in a warm room, either when the amount is to be measured, or the funnel is filled up with snow. A tin vessel of equal area to the funnel may at such times be useful as a subst.i.tute.

Rain gauges are constructed of metal, usually copper, which, besides being readily workable, is little affected by atmospheric influences. If made of iron or zinc, they should be well j.a.panned; if of copper, this is not so essential. The capacity of a gauge should be sufficient to contain at least the probable maximum fall of rain in a day at the locality. Those required for rainy districts must be of large size.

=118. Causes of Rain.=--When the invisible vapour which is diffused in the atmosphere becomes sufficiently cooled, it appears visible as mist or cloud, and a further reduction of temperature causes its precipitation as rain, hail, or snow. The cooling of the higher regions of the atmosphere is doubtless the chief cause of this condensation; but the property which aqueous vapour possesses of radiating heat may also contribute to the result. Moreover, the law which regulates the amount of vapour which air at any particular temperature can sustain in a transparent state, determines that when two bodies of air at different temperatures, saturated with vapour, intermix, some moisture must be rendered visible; and hence, it is not only possible, but highly probable, that rain may result from the conflict of different winds. Let us imagine two cubic yards of air, both saturated with moisture, but having the respective temperatures of 50 and 70 degrees, to come into contact. There will be a tendency to equalize the temperature to a mean, which is 60; and during this process, some of the vapour will be condensed.

For in the air at 50 there is 1107 grains of vapour[11]

and " 70 " 2160 "

------ Total amount of vapour 3267 "

But two cubic yards of air at 60 can only sustain 3132 "

------ Hence there will be deposited 135 " of rain.

It may be conceded, therefore, that when a warm and moist current of air encounters a body of cold air which may not be extremely dry, the mixture is unable to retain the whole of the vapour in an invisible state; so that the excess becomes visible as mist or fog, and, when the temperature has become sufficiently lowered, rain. The British Isles are more or less enveloped in fog, or mist, at the commencement of easterly winds, which, with a sudden change of wind, is exhibited even in summer; while the south-westerly winds, warm, and arriving from the ocean, deposit large quant.i.ties of rain by the cooling effect of the land, colder by reason of its lat.i.tude. When rain occurs with a northerly wind, it is probably due to the deposition from an upper south-westerly current, often apparently proved by the movements of the upper clouds.

=119. Laws of Rain-fall.=--Tropical countries have a dry and a wet season during the year: _dry_, when the sun is at the opposite side of the equator; _wet_, when the sun is overhead. With reference to the British Isles, the statistics collected by Mr. G. J. Symons indicate that: 1st.

The stations of least rain are inland, or on the east or south-east coasts; the stations of greatest rain are on the western coasts. 2nd. The rain-fall is very large in the vicinity of mountain chains or groups, unless the station happens to be some miles to the north-eastward.

It may be well to ill.u.s.trate these remarks by quoting[12] the average fall at a few places, grouping them as--

Westerly.

Inches.

Bodmin 43 Bolton (Lancashire) 44 Coniston (Windermere) 71 Seathwaite 127 Torosay (I. of Mull) 75 Killaloe (Limerick) 38

Central.

Inches.

Enfield 23 Epping 23 Derby 24 York 22 Stirling 39 Perth 29

Easterly.

Inches.

Witham (Ess.e.x) 21 Patrington (Hull) 21 Sunderland 17 Inveresk (Edinburgh) 25 Pittenweem (Fife) 24 Dublin 22

Mr. Green, the celebrated aeronaut, has a.s.serted from his experience, "that whenever a fall of rain happens, and the sky is entirely overcast, there will invariably be found to exist another stratum of cloud at a certain elevation above the former;" and the recent scientific balloon ascents by Mr. Glaisher have tended to confirm this theory. Mr. Glaisher says, "It would seem to be an established fact, that whenever rain is falling from an overcast sky, there is a second stratum above." "It would also seem that when the sky is overcast without rain, that there is no stratum of cloud above, but that the sun is shining on the upper surface.

In every instance in which I have been up under these circ.u.mstances, I have found such to be the case, agreeing in this respect also with Mr.

Green's observations."

The amount of rain collected in a gauge placed near the surface of the earth is larger than in any gauge placed above it; and the higher the gauge is placed, the less water is collected. Mr. Glaisher contends that his balloon experiments corroborate this law.

=120. Utility of Statistics of Rain-fall.=--The utility of knowing the rain-fall of any locality is sufficiently obvious, and little need be said upon the subject. The rain-gauge should be in the hands of every gardener and farmer. In the management of out-door plants and crops, as well as in the construction of cisterns and tanks for the supply of water, a rain gauge is a valuable a.s.sistant. By its use, the gardener will be guided in judging how far the supply of moisture to the earth is needed; and he will also see how beneficial is even a hasty shower to growing plants, when he considers that a fall of rain measuring the tenth of an inch in depth, corresponds to the deposit of about forty hogsheads per acre. The study of the rain-fall of a country is of considerable interest to agriculturists.

The health and increase of domestic animals, the development of the productions of the land, as well as the daily labours of the farmer, are dependent upon the excess or deficiency of rain. "It must be a subject of great satisfaction and confidence to the husbandman to know at the beginning of a summer, by the certain evidence of meteorological results on record, that the season, in the ordinary course of things, may be expected to be a dry and warm one; or to find, in a certain period of it, that the average quant.i.ty of rain to be expected for the month has fallen.

On the other hand, when there is reason, from the same source of information, to expect much rain, the man who has courage to begin his operations under an unfavourable sky, but with good ground to conclude, from the state of his instruments and his collateral knowledge, that a fair interval is approaching, may often be profiting by his observations; while his cautious neighbour who waited 'for the weather to settle' may find that he has let the opportunity go by. This superiority, however, is attainable by a very moderate share of application to the subject; and by the keeping of a plain diary of the barometer and rain-gauge, with the hygrometer and vane, under his daily notice."[13] The statistics of rain-fall are not only valuable and interesting in a meteorological point of view, and for agricultural purposes, but are also highly important in connection with sanitary arrangements for towns, and engineering operations. This is especially evident to the hydraulic engineer. As rain is an important source of water-supply to rivers, ca.n.a.ls, and reservoirs, it is evident that a knowledge of the probable fall for any season or month, at a given place, as furnished by averages of the observations of former years, will be the data upon which the engineer will base his plans for providing for floods or droughts; while the measurement of the actual quant.i.ty which has just fallen, as gathered from the indications of a series of gauges, will suggest to him the precautions to adopt either to economise or conduct away the in-pouring waters.

"When a ca.n.a.l is conducted across an undulating country, its course is necessarily governed by the accidents of the ground, and it alternately rises and falls. In this case, rising by a succession of levels, it necessarily arrives at a certain highest level, which is called by engineers the _summit level_. From this it again descends by a corresponding series of levels. Now, it is evident that, supposing the locks to be all equal in magnitude, the ascent of a vessel will require the descent of as much water from the summit to the lowest level as would fill a single lock; for this quant.i.ty of water must be discharged from each lock of the series when the vessel pa.s.ses through it.

"The same may be said of the process by which the vessel descends along the series of locks on the other side of the summit. It appears, therefore, that a supply of water must always be maintained on the summit level sufficient to fill a single lock twice for each vessel which crosses the summit.

"It happens, fortunately, that by the laws of natural evaporation, rain is precipitated in greater quant.i.ties on elevated summits than on the intermediate valleys, so that the moving power, in this case, accommodates itself to the exigencies of intercommunication."--_Dr. Lardner's "Handbook of Natural Philosophy."_

=121. New Form of Rain-Gauge.=--Since the foregoing pages were in type, a modification of Howard's rain-gauge has been arranged by Mr. Symons, which is compact in design, convenient in use, and low in price. It combines the advantages of most gauges; having solidity, and facility of measurement.

The bottle is placed in a tin case, to the bottom of which are attached stout spikes, which, when forced into the earth, prevent its being upset either by wind or accident. The bottle being transparent, and slits made in the case, the fall of rain is seen at a glance, or with a race-gla.s.s, from a window. The funnel being attached to the cover of the case is thereby kept strictly horizontal, and the depth of rain can be accurately measured by lifting the bottle from its case and emptying it into a graduated gla.s.s jar.

The funnel of this gauge is a very deep cone, to prevent the rain drops outsplashing. When properly placed, the receiving surface will be twelve inches above the ground, which experience has shown to be the most advantageous height.