By contracting the bore of this thermometer, at the bend of the tube, sufficiently to keep the mercury from flowing out of its bulb with too much freedom by motion, the instrument becomes perfectly safe for transmission abroad.

=78. Negretti & Zambra's Second Patent Mercurial Minimum Thermometer.=--In this thermometer a principle is used that has been long known to scientific men, viz. the affinity of mercury for platinum. If mercury be placed in contact with platinum under ordinary circ.u.mstances, no effect will take place; but if the mercury is once made to attack the platinum, the amalgamation is permanent and the contact perfect, so much so, that the principle was made use of in constructing standard barometers. A ring of platinum was fused round the end of the tube, dipping into the mercury; and the contact between the platinum and mercury became so perfect that air could not creep down the tube and up the bore, as in ordinary barometer tubes. This principle of adhesion or affinity of mercury for platinum has been brought into play for the purpose of arresting the mercury after it has reached the minimum temperature in a thermometer.

This thermometer is made as follows:--behind the bulb is placed a supplementary chamber; in the s.p.a.ce or neck between the bulb of the thermometer and the chamber, is placed a small piece of platinum; this may be of any shape or size, but the smaller the better. This is not to fit in the neck; it must, on the contrary, be rather loose; it may be fastened in position or not. The instrument is represented by fig. 59.

[Ill.u.s.tration: Fig. 59.]

_Directions for using._--Having suspended the thermometer in a horizontal position, the mercury is made to stand in exact contact with the platinum plug by slightly elevating the bulb end of the instrument. The thermometer is now ready for observation. On a decrease of temperature, the mercury will endeavour to contract first from the easier pa.s.sage, viz. behind the bulb; but in consequence of the adhesion of the mercury to the platinum, it cannot recede from here, it is therefore forced to contract from the indicating tube, and will continue to do so as long as the temperature decreases; and as no indices are employed in this thermometer, the extreme end of the mercurial column will show "how cold it has been." On an increase of temperature the mercury will glide over the platinum plug and expand by the easier pa.s.sage into the supplementary chamber, and there remain until a decrease of temperature again takes place, when the mercury that had gone into the supplementary chamber will be the first to recede, until it reaches the platinum plug, its further progress being arrested; it will then fall in the indicating tube, and there remain until re-set.

=79. Casella's Mercurial Minimum Thermometer.=--The general form and arrangement of this instrument is shown in fig. 60. A tube with large bore, _a_, has at the end a _flat gla.s.s diaphragm_ formed by the abrupt junction of a small chamber, _b c_, the inlet to which at _b_ is larger than the bore of the indicating tube. The result of this is that on setting the thermometer, as described below, the contracting force of the mercury in cooling withdraws the fluid in the indicating stem only; whilst on its expanding with heat, the long column does not move, the increased bulk of mercury finding an easier pa.s.sage into the small pear-shaped chamber attached.

[Ill.u.s.tration: Fig. 60.]

We believe that a small speck of air must be confined in the chamber, _b c_, to act as a spring to start the mercury from the chamber in the act of setting the thermometer. Were this air not present, the mercury would so adhere to the gla.s.s that no amount of shaking could induce it to flow from the chamber.

_To set the Instrument_, place it in a horizontal position, with the back plate, _d_, suspended on a nail, and the lower part supported on a hook, _e_. The bulb end may now be gently raised or lowered, causing the mercury to flow slowly until the bent part, _a_, _is full_ and the chamber, _b c_, _quite empty_. At this point the flow of mercury in the long stem of the tube is arrested, _and indicates the exact temperature_ of the bulb or air at the time. On an increase of temperature the mercury will expand into the small chamber, _b c_; and a return of cold will cause its recession from this chamber only, until it reaches the diaphragm, _b_. Any further diminution of heat withdraws the mercury down the bore to whatever degree the cold may attain, where it remains until farther withdrawn by increased cold, or till re-set for future observation.

MAXIMA AND MINIMA THERMOMETERS.

=80. Rutherford's= arrangement for obtaining a complete instrument for the registration of heat and cold was simply mounting a maximum thermometer and a minimum thermometer upon the same frame or slab. Thus constructed, they are often called "day and night" thermometers, though somewhat inappropriately; for in temperate climates the temperature of the night sometimes exceeds that of the day, notwithstanding the reverse is the general law of temperature. Fig. 61 will explain the arrangement of Rutherford's day and night thermometer.

[Ill.u.s.tration: Fig. 61.]

=81. Sixe's Self-Registering Thermometer.=--The very ingenious and certainly elegant instrument about to be described was invented by James Sixe, of Colchester. It consists of a long cylindrical bulb, united to a tube of more than twice its length, bent round each side of it in the form of a syphon, and terminated in a smaller, oval-shaped bulb. Figure 62 gives a representation of this instrument. The lower portion of the syphon is filled with mercury; the long bulb, the other parts of the tube, and part of the small bulb, with highly rectified alcohol. A steel index moves in the spirit in each limb of the syphon. The two indices are terminated at top and bottom with a bead of gla.s.s, to enable them to move with the least possible friction, and without causing separation of the spirit, or allowing mercury to pa.s.s easily. They would, from their weight, always rest upon the mercury; but each has a fine hair tied to its upper extremity and bent against the interior of the tube, which acts as a spring with sufficient elasticity to keep the index supported in the spirit in opposition to gravity.

[Ill.u.s.tration: Fig. 62.]

The instrument acts as follows:--A rise of temperature causes the spirit in the long bulb to expand and press some of the mercury into the other limb of the syphon, into which it rises also from its own expansion, and carries the index with it, until the greatest temperature is attained. The lower end of this index then indicates upon the engraved scale the maximum temperature. As the temperature falls the spirit and the mercury contract, and in returning towards the bulb the second index is met and carried up by the mercury until the lowest temperature occurs, when it is left to indicate upon the scale the minimum temperature. The limb of the syphon adjoining the bulb requires, therefore, a descending scale of thermometric degrees; the other limb, an ascending scale. The graduations must be obtained by comparisons with a standard thermometer under artificial temperatures, which should be done in this way for every 5, in order to correct for the inequality in the bore of the tube, and the irregular expansion of the spirit. The instrument is set for observation by bringing the indices into contact with the mercury, by means of a small magnet, which attracts the steel through the gla.s.s, so that it is readily drawn up or down. They should be drawn nearly to the top of the limbs when it is desired to remove the instrument, which should be carefully carried in the vertical position; for should it be inverted, or laid flat, the spirit may get among the mercury, and so break up the column as to require the skill of a maker to put it in order again. For transmission by ordinary conveyances, it requires that attention be given to keep it vertical. The entanglement of a small portion of mercury with the indices is sometimes a source of annoyance in this instrument, for the readings are thereby rendered somewhat incorrect. Small breakages in the mercury, either from intervening bubbles of spirit or adhesion to the indices, may generally be rectified by cautiously tapping the frame of the instrument, so as to cause the mercury to unite by the a.s.sistance thus given to its superior gravity.

These thermometers, when carefully made and adjusted to a standard thermometer, are strongly recommended for ordinary purposes, where strict scientific accuracy is not required. This is also the only fluid thermometer applicable for determining the temperature of the sea at depths.

CHAPTER VIII.

RADIATION THERMOMETERS.

=82. Solar and Terrestrial Radiation considered.=--The surface of the earth absorbs the heat of the sun during the day, and radiates heat into s.p.a.ce during the night. The envelope of gases and vapour, which we call the atmosphere, exerts highly important functions upon these processes.

Thanks to the researches of Professor Tyndall, we are now enabled to understand these functions much more clearly than heretofore. His elaborate, patient, and remarkably sagacious series of experiments upon radiant heat, have satisfactorily demonstrated that _dry_ air is as transparent to radiant heat as the vacuum itself; while air _perfectly saturated_ with aqueous vapour absorbs more than five per cent. of radiant heat, estimated by the thermal unit adopted for the galvanometer indications of the effect upon a thermo-electric pile.

Aqueous vapour, in the form of fog or mist, as is well known, gives to our sensation a feeling of cold, and interferes with the healthy action of the skin and the lungs; the cause being its property of absorbing heat from our person.

Air containing moisture in an invisible state likewise exerts a remarkable influence in radiating and absorbing heat. By reason of these properties, aqueous vapour acts as a kind of blanket upon the ground, and maintains upon it a higher temperature than it would otherwise have. "Regarding the earth as a source of heat, no doubt at least ten per cent. of its heat is intercepted within ten feet of the surface." Thus vapour--whether transparent and invisible, or visible, as cloud, fog, or mist--is intimately connected with the important operations of solar and terrestrial radiation. Cloudy, or humid days, diminish the effect upon the soil of solar radiation; similar nights r.e.t.a.r.d the radiation from the earth. A dry atmosphere is the most favourable for the direct transmission of the sun's rays; and the withdrawal of the sun from any region over which the air is dry, must be followed by very rapid cooling of the soil.

"The removal, for a single summer night, of the aqueous vapour from the atmosphere which covers England, would be attended by the destruction of every plant which a freezing temperature could kill. In Sahara, where 'the soil is fire and the wind is flame,' the refrigeration at night is often painful to bear. Ice has been formed in this region at night. In Australia, also, the _diurnal range_ of temperature is very great, amounting, commonly, to between 40 and 50 degrees. In short, it may be safely predicted, that wherever the air is _dry_, the daily thermometric range will be great. This, however, is quite different from saying that when the air is _clear_, the thermometric range will be great. Great clearness to light is perfectly compatible with great opacity to heat; the atmosphere may be charged with aqueous vapour while a deep blue sky is overhead; and on such occasions the terrestrial radiation would, notwithstanding the 'clearness,' be intercepted." The great range of the thermometer is attributable to the absence of that protection against gain or loss of heat which is afforded when aqueous vapour is present in the air; and during such weather the rapid abstraction of moisture from the surface of plants and animals is very deleterious to their healthy condition. "The nipping of tender plants by frost, even when the air of the garden is some degrees above the freezing temperature, is also to be referred to chilling by radiation." Hence the practice of gardeners of spreading thin mats, of bad radiating material, over tender plants, is often attended with great benefit.

By means of the process of terrestrial radiation ice is artificially formed in Bengal, "where the substance is never formed naturally. Shallow pits are dug, which are partially filled with straw, and on the straw flat pans containing water which had been boiled is exposed to the clear firmament. The water is a very powerful radiant, and sends off its heat into s.p.a.ce. The heat thus lost cannot be supplied from the earth--this source being cut off by the non-conducting straw. Before sunrise a cake of ice is formed in each vessel.... To produce the ice in abundance, the atmosphere must not only be clear, but it must be comparatively free from aqueous vapour."

Considering, therefore, the important consequences attending both terrestrial and solar radiation, it appears to us that observations from radiation thermometers are of much more utility in judging of climate than is usually supposed. These observations are very scanty; and what few are upon record are not very reliable, princ.i.p.ally from bad exposure of the instruments, while the want of uniformity in construction may be another cause. Hersch.e.l.l's actinometer and Pouillet's pyrheliometer, instruments for ascertaining the absolute heating effect of the sun's rays, should, however, be more generally employed by meteorologists. In comparing observations on radiation it should be kept in mind, that "the difference between a thermometer which, properly confined [or shaded], gives the true temperature of the night air, and one which is permitted to radiate freely towards s.p.a.ce, must be greater at high elevations than at low ones;"[6]

because the higher the place, the less the thickness of the vapour-screen to intercept the radiation.

=83. Solar Radiation Thermometer.=--"As the interchange of heat between two bodies by radiation depends upon the relative temperature which they respectively possess, the earth, by the rays transmitted from the sun during the day, must be continually gaining an accession of heat, which would be far from being counterbalanced by the opposite effect of its own radiation into s.p.a.ce. Hence, from sunrise till two or three hours after mid-day, the earth goes on gradually increasing in temperature, the augmentation being greatest where the surface consists of materials calculated, from their colour and texture, to absorb heat, and where it is deficient in moisture, which, by its evaporation, would have a tendency to diminish it."[7] It is, therefore, important to have instruments for measuring the efficacy of solar radiation, apart from those for exhibiting the temperature of the place in the shade.

[Ill.u.s.tration: Fig. 63.]

Fig. 63 shows the arrangement of Negretti & Zambra's maximum thermometer, for registering the greatest heat of the sun's direct rays, hence called a _solar radiation thermometer_. It has a blackened bulb, the scale divided on its own stem, and the divisions protected by a gla.s.s shield. In use it should be placed nearly horizontally, resting on Y supports of wood or metal, with its bulb in the full rays of the sun, resting on gra.s.s, and, if possible, so that lateral winds should not strike the bulb; and at a sufficient distance from any wall, so that it does not receive any _reflected_ heat from the sun. Some observers place the thermometer as much as two feet from the ground. It would be very desirable if one uniform plan could be recognized: that of placing the instrument as indicated in the figure appears to be most generally adopted, and the least objectionable.

=84. Vacuum Solar Radiation Thermometer.=--In order that the heat absorbed by the blackened bulb of the solar radiation thermometer may not in part be carried off by the currents of air which would come into contact with it, the instrument has been improved by Messrs. Negretti and Zambra into the _vacuum solar radiation thermometer_, as ill.u.s.trated by fig. 64.

[Ill.u.s.tration: Fig. 64.]

This consists of a blackened-bulb radiation thermometer, enclosed in a gla.s.s tube and globe, from which all air is exhausted. Thus protected from the loss of heat which would ensue if the bulb were exposed, its indications are from 20 to 30 higher than when placed side by side with a similar instrument with the bulb exposed to the pa.s.sing air. At times when the air has been in rapid motion, the difference between the reading of a thermometer giving the true temperature of the air in the shade, and an ordinary solar radiation thermometer, has been 20 only, whilst the difference between the air temperature and the reading of a radiation thermometer in vacuo has been as large as 50. It is also found that the readings are almost identical at distances from the earth varying from six inches to eighteen inches. By the use of this improvement, it is hoped that the amounts of solar radiation at different places may be rendered comparable; hitherto they have not been so; the results found at different places cannot be compared, as the bulbs of the thermometers are under very different circ.u.mstances as to exposure and currents of air. Important results are antic.i.p.ated from this arrangement. The observations at different places are expected to present more agreement. Observers would do well to note carefully the effect of any remarkable degree of intensity in the solar heat upon particular plants, crops, fruit or other trees.

=85. Terrestrial Radiation Thermometer= is an alcohol minimum thermometer, with the graduations etched upon the stem, and protected by a gla.s.s shield, as shown in figure 65, instead of being mounted on a frame. The bulb is transparent; that is to say, the spirit is not coloured.

[Ill.u.s.tration: Fig. 65.]

In use, it should be placed with its bulb fully exposed to the sky, resting on gra.s.s, the stem being supported by little forks of wood. The precautions required with this thermometer are similar to those for ordinary spirit thermometers, explained at page 76.

[Ill.u.s.tration: Fig. 66.]

=86. aethrioscope.=--The celebrated experimental philosopher, Sir John Leslie, was the inventor of this instrument, the purpose of which is to give a comparative idea of the radiation proceeding from the surface of the earth towards the sky. It consists, as represented in fig. 66, of two gla.s.s bulbs united by a vertical gla.s.s tube, of so fine a bore that a little coloured liquid is supported in it by its own adhesion, there being air confined in each of the bulbs. The bulb, _A_, is enclosed in a highly polished bra.s.s sphere, _D_, made in halves and screwed together. The bulb, _B_, is blackened and placed in the centre of a metallic cup, _C_, which is well gilt on the inside, and which may be covered by a top, _F_. The bra.s.s coverings defend both bulbs from solar radiation, or any advent.i.tious source of heat. When the top is on, the liquid remains at zero of the scale. On removing the top and presenting the instrument to a clear sky, either by night or by day, the bulb, _B_, is cooled by terrestrial radiation, while the bulb, _A_, retains the temperature of the air. The air confined in _B_, therefore, contracts; and the elasticity of that within _A_ forces the liquid up the tube, to a height proportionate to the intensity of the radiation. Such is the sensitiveness of the instrument, that the smallest cloud pa.s.sing over it checks the rise of the liquid. Sir John Leslie says:--"Under a clear blue sky, the _aethrioscope_ will sometimes indicate a cold of fifty millesimal degrees; yet, on other days, _when the air seems equally bright_, the effect is hardly 30." This anomaly, according to Dr. Tyndall, is simply due to the difference in the quant.i.ty of aqueous vapour present in the atmosphere. The presence of invisible vapour intercepts the radiation from the aethrioscope, while its absence opens a door for the escape of this radiation into s.p.a.ce.

=87. Pouillet's Pyrheliometer.=--"This instrument is composed of a shallow cylinder of steel, _A_, fig. 67, which is filled with mercury. Into the cylinder a thermometer, _D_, is introduced, the stem of which is protected by a piece of bra.s.s tubing. We thus obtain the temperature of the mercury.

The flat end of the cylinder is to be turned towards the sun, and the surface, _B_, thus presented is coated with lamp black. There is a collar and screw, _C_, by means of which the instrument may be attached to a stake driven into the ground, or into the snow, if the observations are made at considerable heights. It is necessary that the surface which receives the sun's rays should be perpendicular to the rays; and this is secured by appending to the bra.s.s tube which shields the stem of the thermometer, a disk, _E_, of precisely the same diameter as the steel cylinder. When the shadow of the cylinder accurately covers the disk, we are sure that the rays fall, as perpendiculars, on the upturned surface of the cylinder.

[Ill.u.s.tration: Fig. 67.]

"The observations are made in the following manner:--First, the instrument is permitted, not to receive the sun's rays, but to radiate its own heat for five minutes against an unclouded part of the firmament; the decrease of the temperature of the mercury consequent on this radiation is then noted. Next, the instrument is turned towards the sun, so that the solar rays fall perpendicularly upon it for five minutes; the augmentation of heat is now noted. Finally, the instrument is turned again towards the firmament, away from the sun, and allowed to radiate for another five minutes, the sinking of the thermometer being noted as before. In order to obtain the whole heating power of the sun, we must add to his observed heating power the quant.i.ty lost during the time of exposure, and this quant.i.ty is the mean of the first and last observations. Supposing the letter _R_ to represent the augmentation of temperature by five minutes'

exposure to the sun, and that _t_ and _t_ represent the reductions of temperature observed before and after, then the whole force of the sun, which we may call _T_, would be thus expressed:--_T = R + 1/2(t + t)_.

"The surface on which the sun's rays here fall is known; the quant.i.ty of mercury within the cylinder is also known; hence we can express the effect of the sun's heat upon a given area, by stating that it is competent, in five minutes, to raise so much mercury so many degrees in temperature."--_Dr. Tyndall's "Heat considered as a Mode of Motion."_

[Ill.u.s.tration: Fig. 68.]

=88. Sir John Hersch.e.l.l's Actinometer=, for ascertaining the absolute heating effect of the solar rays, in which _time_ is considered one of the elements of observation, is ill.u.s.trated by fig. 68. The actinometer consists of a large cylindrical thermometer bulb, with a scale considerably lengthened, so that minute changes may be easily seen. The bulb is of transparent gla.s.s filled with a deep blue liquid, which is expanded when the rays of the sun fall direct on the bulb. To take an observation, the actinometer is placed in the shade for one minute and read off; it is then exposed for one minute to sunshine, and its indication recorded; it is finally restored to the shade, and its reading noted. The mean of the two readings in the shade, subtracted from that in the sun, gives the actual amount of expansion of the liquid produced by the sun's rays in one minute of time. For further information, see _Report of the Royal Society on Physics and Meteorology_; or _Kaemtz's Meteorology_, translated by C. V. Walker; or the _Admiralty Manual of Scientific Instructions_.