64. VARIETIES OF THERMOMETERS.
Fig. 37 is an illustration of boxwood scale thermometers for general use and common purposes.
Fig. 38, Negretti and Zambra’s Travelling Thermometer; it is fixed in a plated metal (silver or otherwise) case, similar to a pencil-case, and has the scale divided upon its stem.
Fig. 39, Thermometer mounted on a slab of glass, upon which the scale is etched, the back being either oak, mahogany, or ebony.
Fig. 40, Portable Thermometer, in a bronzed brass or German silver revolving case.
Fig. 41, Pocket Thermometer, on ivory or metallic scale, in morocco or papier-mâché case.
Fig. 42, an Ornamental Drawing-room Thermometer, on ebony or ivory stand, with glass shade.
Fig. 43, representation of highly carved or engine-turned design for thermometer mounts, in ivory or wood, for the drawing-room. Some have the addition of a sundial or compass at the top; they may also be formed for a watch-stand.
Fig. 44,Bath Thermometer, having a float to admit of its being kept in the water.
Fig. 45, Thermometer with ivory scale in glass cylinder, mounted on oak bracket with metal top, for out-door use; as at a window.
Fig. 46, Thermometer for the window, on patent porcelain or glass scale, with oak bracket and convenient brass supports, for placing the instrument at any angle.
Fig. 47,Chemical Thermometer, on boxwood scale, jointed near the bulb on a brass hinge, ranging from 300° to 600°.
Fig. 48,Chemical Thermometer, for acids, graduated on its own stem, suitable for insertion in the tubulure of retorts; they are also made insulated in glass cylinder to protect the graduated stem; ranging from 0° to 600°.
65. Superheated Steam Thermometer.—The great advantage gained by the use of superheated steam in marine and other steam-engines being now generally admitted by engineers, reliable thermometers, reading to 600° at least, are of the utmost importance. To meet this want, Messrs. Negretti and Zambra have constructed for the purpose a substantial form of thermometer, on their patent porcelain scales, in strong and convenient metal mountings, with perforated protection to the bulb. The scales cannot be deteriorated by steam, heat, oil, or dirt; and an occasional wiping will be all that is necessary to keep the divisions and figures clean and visible for any length of time; while careful calibration of the thermometer tubes ensures the most accurate indications attainable. These thermometers are illustrated by figs. 49 & 50. A similar,but cheaper, construction is given to thermometers to be used with hot air, or hot water, apparatus.
66. Thermometer for Sugar Boilingis protected by a metallic frame; and is usually from three to four feet long, the graduations being confined to a space of about twelve inches at the upper part of the instrument, allowing the bulb and greater part of the tube to be immersed in the boiling sugar. The graduations extend to 270° or further. An index is sometimes attached to the scale, which may be set to any degree of heat required to be maintained.
67. EARTH THERMOMETER.
The Earth Thermometer is for ascertaining the temperature of the soil at various depths. It is protected by a brass frame, pointed and strengthened at the end to facilitate insertion into the ground, as in fig. 51.
Fig. 51.
Utility of a Knowledge of the Temperature of the Soil.—The temperature of the soil is an important element in the consideration of climate, as it concerns the vegetable kingdom.
Dr. Daubeny, in hisLectures on Climate, gives the following statement with respect to some temperatures which have been observed just beneath the earth’s surface, in different parts of the globe:—
“The importance of this to vegetation may be estimated by the following considerations:—
“It is known that every plant requires a certain amount of heat, varying inthe case of each species, for the renewal of its growth, at the commencement of the season.
“Now when this degree of heat has spurred into activity those parts that are above ground, and caused them to elaborate the sap, it is necessary that the subterranean portions should at the same time be excited by the heat of the ground to absorb the materials which are to supply the plant with nourishment. Unless the latter function is provided for, the aerial portions of the plant will languish from want of food to assimilate. Indeed, it is even advisable that the roots should take the start of the leaves, in order to have in readiness a store of food for the latter to draw upon.” In another place the professor remarks:—“It has been calculated by Mr. Raikes, from experiments made at Chat Moss, that the temperature of the soil when drained averages 10° more than it does when undrained; and this is not surprising, when we find that 1 lb. of water evaporated from 1,000 lbs. of soil will depress the whole by 10°, owing to the latent heat which it absorbs in its conversion into vapour.”
68. MARINE THERMOMETER.
Fig. 52.
This instrument is a special construction to meet the requirements of navigation. It consists of a carefully constructed thermometer divided on its stem to degrees, which are sufficiently large to admit of subdivision into tenths of degrees by estimation, and ranging from 0° to 130°. The scale is porcelain, having the degrees etched upon it, and burnt-in a permanent black. The instrument is made to slide into a japanned metallic case, for handy use and protection. It is therefore adapted for almost any ordinary purpose; and cannot be injuriously affected by any chemical action arising from air or sea-water. A set of these thermometers consists of six, carefully packed in a neat box; two having japanned metallic cases (fig. 52), the others being designed for use without the case, or to replace a breakage.
This thermometer is employed in the Royal Navy, and for the observations made at sea for the Board of Trade.
The thermometer is now considered a necessary instrument on board ship. Not only is it of invaluable utility in connection with the barometer as a guide to the weather, but its indications are of service in showing the presence of a warm or cold current in the sea; many of the great oceanic currents being characterised by the warmth or coldness of their waters. In seas visited by icebergs, the habitual use of the thermometer would indicate their proximity, as the water is rendered colder for some distance around by the thawing of huge masses of ice. The water over a shoal in the sea is generally colder than the surface-water of the surrounding ocean; which may result from the cold water being brought to the surface by the current of water encountering the shoal. With this fact navigators are well acquainted; and therefore a fall in the sea-water thermometer may forebode that shallow water is at hand. It has been ascertained that fishinhabit regions of the oceans and seas having the peculiar temperature suitable to their habits. The better and firmer sort of fish are found where cold waters exist. Those taken in warmer belts or streams of water, even in the same latitude, are far inferior in condition, and less approved by the palate. The fish of the Mediterranean, a warm sea, are generally poor and scarce. Fish taken in the cold waters between the American shore and the Gulf Stream are much esteemed; while in and on the other side of the stream they are said to be tasteless, and of no flavour. Between the coasts of China and the warm waters of the Japanese current, the seas abound with excellent fish; but in the warm waters of the current and beyond, they are never seen in such shoals.
In fact, it is clearly ascertained that fishes are adapted to climates, like birds and beasts. It has been even affirmed, after careful investigation, that herrings, which abound in the British Seas, and form a most important branch of our fisheries, can only be found in a temperature varying from 54° to 58°. Hence the thermometer, if brought into use by the fishermen, would guide them to the spots where they may with the best chance cast their nets on dark nights, when other indications are not perceptible.
This thermometer in its metallic case is perfectly suited for dipping overboard, or placing in a bucket of water just taken from the sea, to ascertain its temperature.
SELF-REGISTERING THERMOMETERS.
69. Importance of Self-Registering Thermometers.—Heat being apparently the most effective agent in producing meteorological phenomena, the determination of the highest temperature of the day, and the lowest during the night, is a prime essential to enable an estimate of the climate of any place to be formed. To observe these extremes by means of the ordinary thermometer would be impracticable, from the constant watchfulness which would be necessary. Hence, the utility and importance of self-recording thermometers are evident. A thermometer constructed toregisterthe highest temperature is usually called amaximum thermometer; one to show the lowest temperature is termed aminimum thermometer; and if made to record both extremes of temperature, it is designated amaximum-and-minimum thermometer. We will, for the sake of method, describe the instruments in use in this order.
It would carry us beyond our scope to explain in detail the methods of dealing with temperature observations; but we may remark that half the sum of the maximum and minimum temperature of each day of twenty-four hours, is not what meteorologists designate themean daily temperature, although it very frequently approximates to it. The mean temperature of the day is understood to be the average of twenty-four consecutive hourly readings of a thermometer; and meteorology now supplies formulæ whereby this result can be deduced from two or three observations only in a day. But we would observe that the actual mean temperature of any place has not such an important influence upon life, either animal or vegetable, as the abruptness and magnitude of the variations of temperature. Climate, therefore, should be estimated more by the range of the thermometer than by the average of its indications. The Registrar General’s returns prove that with a wide range of the thermometer, the mortality greatly increases; and it is now becoming apparent to meteorologists that the daily range of the thermometer marks the effects of temperature on the health of men, and the success of crops, better than any other meteorological fact of which we take cognizance. Now that self-registering thermometers are constructed with mercury, the most appropriate of all thermometric substances, not only for maxima, but likewise for minima temperatures, the determination of the diurnal range of temperature is rendered more certain, and observations at different places are more strictly comparable.
MAXIMA THERMOMETERS.
70. Rutherford’s Maximum Thermometer.—The maximum thermometer, invented by Dr. John Rutherford, differs from an ordinary thermometer in having a small cylinder of steel, porcelain, or aluminium, moving freely in the tube beyond the mercury, so as to form an index. The stem of the thermometer is fixed horizontally on the frame, which must be suspended in the same position, as represented in fig. 53. The instrument is set by holding it bulb downward, so as to allow the index to fall by its own gravity into contact with the mercury. Increase of heat produces expansion of the mercury, which consequently pushes forward the index. When the temperature decreases, the mercury recedes from the index, leaving it so that the extremity which was in contact with the mercury indicates upon the scale the highest temperature since the instrument was last set.
Fig. 53.
As it is easily constructed and is comparatively cheap, it is still employed for ordinary purposes. Its disadvantages are, firstly, its liability of soon getting out of order by the index becoming embedded in the mercury, or fixed by oxidation, thus rendering it altogether useless; secondly, the ease with which the index can be displaced by the wind moving the instrument, or other accidental disturbance, so as to cause it to give erroneous indications occasionally; and thirdly, its consequent total unfitness for use at sea.
In the part of the tube beyond the mercury, a small quantity of air is enclosed for the purpose of preventing the metal flowing freely in the tube. This necessitates the construction of a larger bulb, which renders the thermometer less sensitive. Moreover, as it frequently happens that some mercury passes the index, particles of air insinuate themselves in the metal, and cause separations in the column, which very often can be removed only by a maker. To facilitate this re-adjustment, a small chamber is left at the end of the tube, and the mercury being expanded into it by heat until the index and air bubbles are forced into it, if possible, upon the cooling down again, by a little management, the mercury will contract, leaving the air and index behind. Yet sometimes the index cannot be moved in the least from its place of fixture, so that the instrument must be virtually reconstructed.
71. Phillip’s Maximum Thermometer.—A maximum thermometer, better perhaps in its action than Rutherford’s, has been suggested by Professor John Phillips, of Oxford. A small portion of air is introduced into an ordinary thermometer, so as to cut off about half an inch of the mercurial thread near its end in the tube. This forms a maximum thermometer, when the stem is arranged horizontally. The isolated portion is pushed forward by expansion, and is left in this position when themercury contracts. The end remote from the bulb shows on the scale the maximum temperature.
When made with a capillary tube so fine that the attraction arising from capillarity overcomes the force of gravity, and prevents the mercury falling to the end of the tube when the instrument is inverted, it forms a very serviceable thermometer, quite portable and suitable for use on board ship. In such a tube a smart shake from a swing of the hand is required to bring the detached portion back to the column, so as to set the instrument for future observation; no ordinary motion will move it. When the thermometer has not this peculiarity, the mercury will flow to the end, if held bulb downward; and in this state it is not at all a satisfactory instrument, as the air is likely to be displaced, and a great deal of tact is requisite to again get it to divide the column suitably. It has been found in practice that the air bubble at different temperatures assumes different lengths, and if very small it disappears in a few years by oxidation and by diffusion with the mercury, so that the instrument becomes defective and uncertain in action,—results which led to the construction of the self-registering mercurial maximum thermometer, invented and patented by Messrs. Negretti and Zambra. It has been before the public about twelve years; we may therefore, now, safely speak of its merits.
72. Negretti and Zambra’s Patent Maximum Thermometerconsists of a glass tube containing mercury fitted on an engraved scale, as shown in fig. 54. The part of the thermometer tube above the mercury is entirely free from air; and at the pointAin the bend above the bulb, is inserted and fixed with the blow-pipe a small piece of solid glass, or enamel, which acts as a valve, allowing mercury to pass on one side of it when heat is applied, but not allowing it to return when the thermometer cools. When mercury has been once made to pass the contraction, which nothing but the expansive force of heat can effect, and has risen in the tube, the upper end of the column registers the maximum temperature. To return the mercury to the bulb, we must apply a force equal to that which raised it in the tube; the force employed is gravity, assisted when necessary by a little agitation of the instrument.
Fig. 54.
The degrees are generally divided on the stems of these thermometers, but their frames of course bear a scale as well. The makers have various styles of framing in wood, metal, porcelain, and even glass. Each material is eligible according to requirements. Porcelain scales, having the marksetchedupon them by acid and permanently blackened and baked in,—by a process for which theinventors have a separate patent,—will be found very serviceable, as they do not corrode or tarnish by exposure to any kind of weather; while any amount of dust and dirt can readily be cleaned off.
The chief recommendation of this thermometer is its simplicity of construction, enabling it to be used with confidence and safety. Of no other maximum thermometer can it be said that it is impossible to derange or put it out of order; hence, as regards durability, it surpasses all others. Nothing short of actual breakage can cause it to fail. Hence it is the most easily portable of all self-registering thermometers, an advantage which renders it suitable for travellers, and for transmission abroad. In the year 1852, the British Meteorological Society reported this thermometer to be “the best which has yet been constructed for maximum temperature, and particularly for sun observations.” Since then eleven years have elapsed, and it is still without a rival.
Directions for use.In using this thermometer for meteorological observations, it should be suspended by means of two brass platesB,C, attached for that purpose, in such manner that it hangs raised up a little atC, and so placed that it is in the shade, with the air passing freely to it from all sides; then, on an increase of heat, the mercury will pass up the tube as in an ordinary thermometer, and continue doing so as long as the heat increases. On a decrease of heat, the contraction of mercury will take placebelowthebendin the tube, leaving the whole column of mercury in the tube, thus registering the highest temperature, and showing such till the instrument is disturbed.
To prepare the instrument for future observations, remove and hold it perpendicularly, with the bulb downward, and then shake it. The mercury will then descend in the tube, and indicate the temperature of the air at that time; and, when again suspended, is prepared for future observation.
After the temperature has attained a maximum, there will be, with a decrease of heat, a slight contraction of mercury in the tube—as well as of that in the bulb—and hence doubts have arisen as to the accuracy of the registration; but calculation shows, and critical trial has proved, that the greatest daily range of temperature will not produce an error large enough to be appreciable on the scale.
A very great advantage of this thermometer is that the mercury may be allowed to flow to the end of the tube without the maximum temperature attained during an experiment being lost. It can be employed with the bulb uppermost. All that is necessary for reading the maximum temperature is to slope the instrument so that the mercury flows gently towards the bulb. It will then stop at the contraction so as to show the maximum temperature on the scale. Afterwards the mercury is driven into the bulb by agitating the instrument while held in the hand. Hence the instrument is invaluable as a registering thermometer on board ship, as its indications are in no way affected by the motions and tremors of the vessel.
For physiological experiments, such as taking the temperature of the mouthin fever, this thermometer is the only one that can be used with certainty, as it can be held in any position, without losing the maximum temperature attained.
MINIMA THERMOMETERS.
73. Rutherford’s Alcohol Minimum Thermometer, fig. 55, consists of a glass tube, the bulb and part of the bore of which is filled with perfectly pure spirits of wine, in which moves freely a black glass index. A slight elevation of the thermometer, bulb uppermost, will cause the glass index to flow to the surface of the liquid, where it will remain, unless violently shaken. On adecreaseof temperature the alcohol recedes, taking with it the glass index; on anincreaseof temperature the alcohol alone ascends in the tube, leaving the end of the indexfarthestfrom the bulb indicating the minimum temperature.
Fig. 55.
Directions for using, &c.—Having caused the glass index to flow to the end of the column of spirit, by slightly tilting the thermometer, bulb uppermost, suspend the instrument (in the shade with the air passing freely to it on all sides) by the two brass plates attached for that purpose,—in such manner that the bulb is about half an inch lower than the upper, or the end of the thermometer farthest from the bulb; then, on a decrease of temperature, the spirits of wine will descend, carrying with it the glass index; on an increase of temperature, however, the spirits of wine will ascend in the tube, leaving that end of the small glass index farthest from the bulb indicating the minimum temperature. To reset the instrument, simply raise the bulb end of the thermometer a little, as before observed, and the index will again descend to the end of the column, ready for future observation.
Precautions.—1. By no means jerk or shake an alcohol minimum thermometerwhen resettingit, for by so doing it is liable to disarrange the instrument, either by causing the index to leave the spirit, or by separating a portion of the spirit from the main column.
2. As alcohol thermometers have a tendency to read lower by age, owing to the volatile nature of the fluid allowing particles in the form of vapour to rise and lodge in the tube, it becomes necessary to compare them occasionally with a mercurial thermometer whose index error is known; and if the difference be more than a few tenths of a degree, examine well the upper part of the tube to see if any alcohol is hanging in the bore thereof; if so, the detached portion of it can be joined to the main column by swinging the thermometer with a pendulous motion,bulb downwards.
3. The spirit column is sometimes much separated by jolting in travelling. If the instrument is in such a condition when received, it should be held by the righthand, bulb downward, and the frame tapped smartly, but cautiously, against the palm of the left hand. The broken thread of spirit will soon begin to join, and by continuing the operation a sufficient time all the bubbles will disappear, and the thermometer become as good as ever.
74. Horticultural Minimum Thermometer.—This instrument, represented in fig. 56, is a special construction of Rutherford’s minimum thermometer to meet the requirements of horticulturists. It is desirable, if not essential, that gardeners should have the means of ascertaining to what temperature stoves and greenhouses descend on cold nights, especially in winter. This thermometer is mounted on a strong cast zinc frame, with the divisions and figures of the scale raised.
Fig. 56.
The sunk surface of the frame is painted dark; the figures and division a bright colour, so that observations can be made without a close inspection of the instrument.
The directions for using are the same as those given in the preceding section. It may be used as an ordinary thermometer, by simply hanging it from the top loop, in which position, the coloured liquid will always indicate the present temperature.
It was a source of annoyance with the ordinary boxwood and flat metal scales, that after a time, exposure to a damp warm atmosphere favoured the growth of confervæ upon them, and obliterated the divisions; the plan of raising the figures and divisions of the scale has been found to prevent the destruction of the instrument in this way.
75. Baudin’s Alcohol Minimum Thermometer.—This instrument resembles Rutherford’s thermometer in appearance; its indications are given by the expansion and contraction of alcohol, and its minimum temperature is likewise registered by a glass index being pulled back and left behind by the alcohol, as in Rutherford’s instrument. There is, however, a great improvement in Baudin’s instrument; for whilst Rutherford’s thermometer can only register in a horizontal position, Baudin’s can be used either horizontally or vertically, as necessity may require. This important change is effected in the following manner:—Instead of the index in the thermometer being loose and free to run up and down according to the position in which the instrument is held, as in Rutherford’s, the index in the new instrument is made to fit the bore of the tube as nearly tight as possible, so much so that in holding the thermometer even upside down, or shaking it, the index will not shift from its position; but, inasmuch as a minimum thermometer with an immoveable index could not be set when required for observation, and would consequently beuseless, the inventor has introduced behind the index a piece of solid glass, about one-and-a-half inch in length, which moves freely in the alcohol. The addition of the weight of this piece of glass on the top of the index, when turned upside down, forces the index down to the edge of the alcohol; and it is there left, as in the case of the ordinary Rutherford’s thermometer. It is, therefore, by turning the thermometer upside down, and letting the moveable piece of glass fall on the index, that the index is driven to the end of the alcohol; after this operation the thermometer is hung up either horizontally or vertically, and will then be ready for use.
The index, although immoveableper se, is by the alcohol drawn back, as in the ordinary minimum, and its indications are read off on the scale from the top of the index.
76. Mercurial Minima Thermometers desirable.—Alcohol does not expand equally for equal increments of heat, consequently errors are likely to exist in the scale indications unless the graduations are very accurately—not necessarily equally—made. On this account, as well as from the volatility of alcohol, and the intervention of gaseous partitions in the tube, a good and thoroughly reliable minimum thermometer was for a long time a desideratum. It was desirable to obtain a thermometer which should register the lowest temperature by mercury, the fluid in general use for meteorological thermometers. Several instruments have recently been invented to meet this requirement, which are suitable and satisfactory for land purposes, but one well adapted for use on board ship is still very much wanted.
For very low temperatures, alcohol thermometers will always be required; as mercury freezes at -40° F, and contracts very irregularly much before this point, while alcohol has never yet been frozen.
Fig. 57.
77. Negretti and Zambra’s Patent Mercurial Minimum Thermometer, represented by fig. 57, has a cylindrical bulb of large size, which, at first sight, might induce the idea that the instrument would not be sufficiently sensitive; but as length is given to the cylinder instead of increasing its diameter, it will be found as sensitive as a globular bulb of the same diameter, and much more so than an ordinary alcohol thermometer.
The reason for having the bulb large is to allow the internal diameter of the thermometer tube to be larger than that generally used for thermometrical purposes, so that a steel index, pointed at both ends, may move freely within when required.
The tube is blown, filled and regulated in the usual way, 60° of temperature being about half-way up the tube. A small cylindrical bulb is then formed at the upper end of the tube, and then is introduced a steel needle pointed at both ends, that in contact with the mercury being abrupt, the other more prolonged. The openextremity of the tube is now drawn out into a fine capillary tube, and the bulb of the instrument warmed so as to cause the mercury to fill the tube completely. When the mercury reaches the capillary tube, the flame of a blow-pipe is applied; the glass is dexterously melted, the superfluous part taken away, and the tube left hermetically closed. During this operation, the steel index has been embedded in the heated mercury. As the instrument cools, if held upright, the mercury will recede and expose the needle, which will then follow the descending column simply by its own gravity. In this condition the thermometer resembles Rutherford’s maximum, being a tube of mercury with a steel index floating on its surface; but it possesses these important advantages: it is quite free from air, so that the mercury can move with perfect freedom; and the index is pointed at both ends, to allow the mercury to pass, instead of being ground flat to prevent it.
Fig. 58.
To use the Thermometer, it is suspended perpendicularly (figure 57) with the steel index resting on the surface of the mercurial column. As the mercury in the cylinder contracts, that in the tube descends, and the index, of its own gravity, follows it; on the contrary, as the mercury expands and rises in the tube, it passes the index on one side, and in rising, exerts a lateral pressure on the needle, and jams it to one side of the tube, where it remains firmly fixed, leaving the upper point of the needle indicating the minimum temperature. In this thermometer, the reading is always from the upper point of the needle, and not from the mercury itself.
To extricate the Needlefrom the mercury, a magnet is used, when, if the needle is embedded only a few degrees, it can readily be withdrawn without altering the position of the instrument. Should the magnet not be sufficient for the purpose, we simply turn the thermometer on its support from the upright position, slightly elevating the bulb (fig. 58 (2)). The mercury and index will then flow into the small reservoir. Should the index not freely leave the tube with the mercury, assist it with a magnet, and when the mercury and index are in the upper bulb (figure2), apply a magnet outside, which will attract and hold fast the index; and whilst thus holding it, again bring the thermometer to the upright position, when the mercury will immediately fall back into the tube, leaving the index attached to the magnet (figure4), with which it is guided down to the surface of the mercury, ready for another observation.
Care must be taken not to withdraw the magnet until the index is in contact with mercury; for, if released before touching, it might plunge too deeply, and give a false indication. The rule for re-setting it will be to bring the needle-point in contact with the mercury, and then withdraw the magnet, having previously ascertained that no particles of mercury are attached to the index.
It may sometimes, though rarely, happen, that from the time aminimum temperature is registered by the index, and by the time an observation is made, the mercury may have risen so high in the tube as to completely pass the index, as shown (figure3). Should it so happen, the space which the index occupies will readily be observed, as it will be pressed to one side of the tube, causing a different appearance in that part, although the point of the needle may not be seen. If such be the case, apply a magnet to the spot where you see the index is fixed: this will hold the needle firmly. Then, by slightly tilting the thermometer bulb uppermost, the mercury will flow into the top bulb, leaving the index attached to the magnet, and quite uncovered. Having taken the reading, draw the needle into the top bulb, and hold it there whilst you adjust the thermometer by again bringing it to the upright position.
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 circumstances, 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 space 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.
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 passage, viz. behind the bulb; but in consequence of the adhesion of themercury 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 passage 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 aflat glass diaphragmformed by the abrupt junction of a small chamber,b c, the inlet to which atbis 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 passage into the small pear-shaped chamber attached.
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 glass 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 fulland 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 temperatureof 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’sarrangement 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.
Fig. 61.
Fig. 62.
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 glass, to enable them to move with the least possible friction, and without causing separation of the spirit, or allowing mercury to pass 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.
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 ofthermometric 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 glass, 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 assistance 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.
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 space 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 thatdryair is as transparent to radiant heat as the vacuum itself; while airperfectly saturatedwith 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 retard 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, thediurnal rangeof temperature is very great, amounting, commonly, to between 40 and 50 degrees. In short, it may be safely predicted, that wherever the air isdry, the daily thermometric range will be great. This, however, is quite different from saying that when the air isclear, 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 terrestrialradiation 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 space. 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, principally from bad exposure of the instruments, while the want of uniformity in construction may be another cause. Herschell’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 space, 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 space. 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 inmoisture, 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.
Fig. 63.