Fig. 63 shows the arrangement of Negretti & Zambra’s maximum thermometer, for registering the greatest heat of the sun’s direct rays, hence called asolar radiation thermometer. It has a blackened bulb, the scale divided on its own stem, and the divisions protected by a glass 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 grass, 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 anyreflectedheat 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 thevacuum solar radiation thermometer, as illustrated by fig. 64.
Fig. 64.
This consists of a blackened-bulb radiation thermometer, enclosed in a glass 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 passing air. At times when the air has been in rapid motion, the difference between the reading of a thermometer giving the true temperature of theair 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 circumstances as to exposure and currents of air. Important results are anticipated 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 Thermometeris an alcohol minimum thermometer, with the graduations etched upon the stem, and protected by a glass 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.
Fig. 65.
In use, it should be placed with its bulb fully exposed to the sky, resting on grass, the stem being supported by little forks of wood. The precautions required with this thermometer are similar to those for ordinary spirit thermometers, explained atpage 76.
Fig. 66.
86. Æthrioscope.—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 glass bulbs united by a vertical glass 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 brass 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 brass coverings defend both bulbs from solar radiation, or any adventitious 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 inB, therefore, contracts; and the elasticity of that withinAforces 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 passing over it checks the rise of the liquid. Sir John Leslie says:—“Under a clear blue sky, theæthrioscopewill 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 quantity of aqueous vapour present in the atmosphere. The presence of invisible vapour intercepts the radiation from the æthrioscope, while its absence opens a door for the escape of this radiation into space.
Fig. 67.
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 brass 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 brass 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.
Fig. 68.
“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 powerof the sun, we must add to his observed heating power the quantity lost during the time of exposure, and this quantity is the mean of the first and last observations. Supposing the letterRto represent the augmentation of temperature by five minutes’ exposure to the sun, and thattandt¹represent the reductions of temperature observed before and after, then the whole force of the sun, which we may callT, would be thus expressed:—T = R + ½(t + t¹).
“The surface on which the sun’s rays here fall is known; the quantity 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.”
88. Sir John Herschell’s Actinometer, for ascertaining the absolute heating effect of the solar rays, in whichtimeis considered one of the elements of observation, is illustrated 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 glass 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, seeReport of the Royal Society on Physics and Meteorology; orKæmtz’s Meteorology, translated by C. V. Walker; or theAdmiralty Manual of Scientific Instructions.
DEEP-SEA THERMOMETERS.
89. On Sixe’s Principle.—Thermometers for ascertaining the temperature of the sea at various depths are constructed to register either the maximum or minimum temperature, or both. The principle of each instrument is that of Sixe. There are very few parts of the ocean in which the temperature below is greater than at the surface, except in the Polar Seas, where it is generally found to be a few degrees warmer at considerable depths than at the surface. When the instrument is required to register only one temperature, it can be made narrower and more compact—a great advantage in sounding; and with less length of bulb and glass tube, so that the liability of error is diminished. Hence, the minimum is the most generally useful for deep-sea soundings. These thermometers must be sufficiently strong to withstand the pressure of the ocean at two or three miles of depth, where there may be a force exerted to compress them exceeding three or four hundred atmospheres (of 15 lbs. to the square inch).
Many have been the contrivances for obtaining correct deep-sea indications. Thermometers and machines of various sorts have been suggested, adopted, and eventually abandoned as only approximate instruments. The principal reason for such instruments failing to give correct or reliable indications, has been that the weight or pressure on the bulbs at great depths has interfered with the correct reading of the instruments. Thermometers have been enclosed in strong water-tight cases to resist the pressure; but this contrivance has only had the tendency to retard the action, so much so as to throw a doubt on the indications obtained by the instrument so constructed.
The thermometers constructed by Messrs. Negretti and Zambra for this purpose do not differ materially from those usually made under the denomination of Sixe’s thermometers, except in the following most important particular:—The usual Sixe’s thermometers have a central reservoir or cylinder containing alcohol; this reservoir, which is the only portion of the instrument likely to be affected by pressure, has been, in Negretti and Zambra’s new instrument, superseded by a strong outer cylinder of glass, containing mercury and rarefied air; by this means the portion of the instrument susceptible of compression, has been so strengthened that no amount of pressure can possibly make the instrument vary. This instrument has been tested in every possible manner, and the results have been highly satisfactory, so much so as to place their reliability beyond any possible doubt.
Fig. 69.
The scales are made of porcelain, and are firmly secured to a back of oak, which holds in a recess the bulb with its protecting shield, and is rounded off so as to fit easily and firmly in a stout cylindrical copper case, in which the thermometer is sent down when sounding (see fig. 69). The lid of the case is made to fit down closely, and water-tight. At the bottom of the case is a valve opening upward; and the lid has a similar valve. These allow the water to pass through the case as the instrument sinks, so that the least amount of obstruction is offered to the descent. At the lower end of the case is a stout brass spring, to protect the instrument from a sudden jar if it should touch the bottom while descending rapidly. As the instrument is drawn up, the valves close with the weight of water upon them, and it arrives at the surface filled with water brought up from its lowest position. The deep-sea thermometers used in the Royal Navy are of this pattern.
90. Johnson’s Metallic Deep-Sea Thermometer.—The objection to the employment of mercurial thermometers for ascertaining the temperature of the ocean at depths, arising from the compression of the bulbs, which was of such serious consequence previous to the modification made in the construction of the instrument by Messrs. Negretti and Zambra, led to the construction of a metallic thermometer altogether free from liability of disturbance from compression by the surrounding water; which, however, is certainly not so sensitive to changes of temperature as mercury. This instrument is the invention of Henry Johnson, Esq., F.R.A.S., and is thus described by him:—
“During the year 1844 some experiments were made by James Glaisher, Esq., F.R.S., on the temperature of the water of the Thames near Greenwich at the different seasons of the year; when that gentleman found that the indications of temperature were greatly affected by the pressure on the bulbs of the thermometers. At a depth of 25 feet this pressure would be nearly equal to the presence of three-fourths of an atmosphere. These observations demonstrate the importance of using in deep-sea soundings an instrument free from liability of disturbance from compression by the surrounding water, and have ultimately led to the construction of the thermometer now to be described.
“The instrument is composed of solid metals of considerable specific gravity, viz. of brass and steel, the specific gravity of these metals being 8·39 and 7·81 respectively. They are therefore not liable to compression by the water, which under a pressure of 1,120 atmospheres, or at a depth of 5,000 fathoms in round numbers, acquires a density or specific gravity of 1·06. In the construction of this instrument, advantage has been taken of the well-known difference in the ratios of expansion and contraction by heat and cold of brass and steel, to form compound bars of thin bars of these metals riveted together; and which will be found to assumea slight curve in one direction when heat has expanded the brass more than the steel, and a slight one in the contrary direction when cold has contracted the brass more than the steel.
Fig. 70.
“The indications of the instrument record the motions under changes of temperature of such compound bars; in which the proportion of brass, the more dilatable metal, is two-thirds, and of steel one-third.
“Upon one end of a narrow plate of metal about a foot long,a, are fixed three scales of temperature,h, which ascend from 25° to 100° F., and which are shown more clearly in the drawing detached from the instrument. Upon one of these scales the present temperature is shown by the pointer,e, which turns upon a pivot in its centre. The register index,g, to the maximum temperature, and the index,f, to the minimum temperature, are moved along the other scales by the pin upon the moving pointer, ate, where they are retained by stiff friction. At equal distances from the centre of the pointer are two connecting pieces,d d, by which it is attached to the free ends of two compound bars,b b, and its movements correspond with the movements of the compound bars under variations of temperature. The other ends of the bars are fastened by the plate,c, to the plate,a, on which the scales of temperature are fixed. The connection of the bars with both sides of the centre of the pointer prevents disturbance of indication by lateral concussion. The case of the instrument has been improved at the suggestion of Admiral FitzRoy, and now presents to the water a smooth cylindrical surface, with rounded ends, and without projection of fastenings.
“In surveying expeditions, this instrument would be found useful in giving notice of variation of depth of water, and of the necessity for taking soundings. A diminution of the temperature of water has been observed by scientific voyagers to accompany diminution of depth, as on nearing land, or approaching hidden rocks or shoals. Attention would also thus be attracted to the vicinity of icebergs.”
Fig. 71.
This thermometer might easily be modified to serve for several other important purposes, such as the determination of the temperature of intermittent hot springs, and mud volcanoes.
The principle of this thermometer is not altogether new; but the duplicate arrangement of the bars, which effectually prevents the movement of the indices by any shaking, and the application are certainly novel. Professor Trail, in theLibrary of Useful Knowledge, writes:—“In 1803, Mr. James Crighton, of Glasgow, published a new‘metallic thermometer,’ in which the unequal expansion of zinc and iron is the moving power. A bar is formed by uniting a plate of zinc (fig. 71),c d, 8 inches long, 1 inch broad, and ¼ inch thick, to a plate of iron,a b, of the same length. The lower extremity of the compound bar is firmly attached to a mahogany board ate e; a pin,f, fixed to its upper end, plays in the forked opening in the short arm of the index,g. When the temperature is raised, the superior expansion of the zinc,c d, will bend the whole bar, as in the figure; and the index,g, will move along the graduated arc, from right to left, in proportion to the temperature. In order to convert it into aregister thermometer, Crighton applied two slender hands,h h, on the axis of the index; these lie below the index, and are pushed in opposite directions by the stud,i,—a contrivance seemingly borrowed from the instrument of Fitzgerald,” a complicated metallic thermometer, described by the Professor previously.
BOILING-POINT THERMOMETERS.
91. Ebullition.—The temperature at which a fluidboilsis called theboiling-pointof that particular fluid. It is different for different liquids; and, moreover, in the same liquid it varies with certain changes of circumstance. Thus the same liquid in various states of purity would have its boiling temperature altered in a slight degree. There is also an intimate connection with the pressure under which a fluid is boiled, and its temperature of ebullition. Liquids boiled in the open air are subjected to the atmospheric pressure, which is well known to vary at different times and places; and the boiling-point of the liquid exhibits corresponding changes. When the pressure is increased on the surface of any fluid, the temperature of ebullition rises; and with a decrease of pressure, the boiling goes on at a lower degree of heat.
In the case of water, we commonly state the boiling-point to be 212° F.; but it is only so at the level of the sea, under the mean pressure of the atmosphere, represented, in the latitude of London, by a column of 29·905 inches of mercury, at a temperature of 32° F., and when the water is fresh and does not contain any matter chemically dissolved in it. When steam is generated and confined in a boiler, the pressure upon the boiling water may be several times greater than that of the atmosphere. Experimentally it has been found, that if the pressure in the boiler be 25 lbs. on the square inch, the temperature of the boiling water, and of the steam likewise, is raised to 241°; while under the exhausted receiver of an air-pump, water will boil at 185°, when the pressure is reduced to 17 inches of mercury.
92. Relation between the Boiling-Point and Elevation.—Now, as the atmospheric pressure is diminished by ascent, as shown by the fall of mercury in the barometer, it follows that in elevated localities water, or any other fluid, heated in the open air, will boil at a temperature lower than at the sea-level. Therefore, there must be some relation between the height of a hill, or mountain, and the temperature at which a fluid will boil at that height. Hence, the thermometer, as used to determine the boiling-point of fluids, is also an indicator of the atmospheric pressure; and may be used as a substitute for the barometer in measuring elevations.
If the atmospheric pressure were constant at the sea-level, and always the same for definite heights, we might expect the boiling-points of fluids also to be in exact accordance with height; and the relation once ascertained, we could readily, by means of the thermometer and boiling water, determine an unknown height, or for a known elevation assert the boiling temperature of a liquid. However, as the atmospheric pressure is perpetually varying at the same place, within certain limits, so there are, as it were, sympathetic changes in the boiling temperatures of fluids.It follows from this, that heights can never be accurately measured, either by the barometer or the boiling-point thermometer, by simply observing at the places whose elevations are required. To determine a height with any approach to accuracy, it is necessary that a similar observation should be made at the same time at a lower station, not very remote laterally from the upper, and that they should be many times repeated. When such observations have been very carefully conducted, the height of the upper station above the lower may be ascertained with great precision, as has been repeatedly verified by subsequent trigonometrical measurement of elevations so determined. If the lower station be at the sea-level, of course the absolute height of the upper is at once obtained.
93. Mountain Thermometer; sometimes called Hypsometric Apparatus.—We have now to examine the construction of the boiling-point thermometer, and its necessary appendages, as adapted for the determination of heights.
Fig. 72.
Fig. 73.
Messrs. Negretti and Zambra’s arrangement of the instrument is shown in figures 72 and 73.
The thermometer is made with an elongated bulb, so as to be as sensitive as possible. The scale, about a foot long, is graduated on the stem, and ranges from 180° to 214°, each degree being sufficiently large to show the divisions of tenths of a degree. A sliding metallic vernier might perhaps with advantage be attached to the stem, which would enable the observer to mark hundredths of a degree; which, however, he can pretty well do by estimation. The boiler is so contrived as to allow, not only the bulb, but the stem also of the thermometer, to be surrounded by the steam. The arrangement is readily understood by reference to the accompanying diagram, fig. 73.
C, is a copper boiler, supported by a tripod stand so as to allow a spirit-lamp,A, made of metal to be placed underneath. The flame from the lamp may be surrounded by a fine wire gauze,B, which will prevent it being extinguished when experimenting in the external air.E E E, is a three-drawn telescope tube, proceeding from the boiler, and open also at top. Another tube, similarly constructed, envelops this, as shown byD D D. This tube is screwed to the top of the boiler, and has two openings, one at the top to admit the thermometer, the other lowdown,G, to give vent to the steam. As the steam is generated, it rises in the inner tube, passes down between the tubes, and flows away atG. The thermometer is passed down, supported by an india-rubber washer, fitting steam tight, so as to leave the top of the mercury, when the boiling-point is attained, sufficiently visible to make the observation. The telescopic movement, and the mode of supporting the thermometer, enable the observer always to keep the bulb near the water, and the double tube gives all the protection required to obtain a steady boiling-point. Some boiling-point thermometers are constructed with their scales altogether exposed to the air, which may be very cold, and consequently may contract to some extent the thread of mercury outside the boiler. The steam, having the same temperature as the boiling water, keeps the tube, throughout nearly its whole length, at the same degree of heat, in the apparatus described. The whole can be packed in a tin case very compactly and securely for travelling, as in fig. 72.
Directions for Using.—When the apparatus is required for practical use, sufficient water must be poured into the boiler to fill it about one third, through an opening,F, which must be afterwards closed by the screw plug. Then apply the lighted lamp. In a short time steam will issue fromG; and the mercury in the thermometer, kept carefully immersed, will rise rapidly until it attains a stationary point, which is the boiling temperature. The observation should now be taken and recorded with as much accuracy as possible, and the temperature of the external air must be noted at the same time by an ordinary thermometer.
The water employed should be pure. Distilled water would therefore be the best. If a substance is held mechanically suspended in water, it will not affect the boiling-point. Thus, muddy water would serve equally as well as distilled water. However, as it cannot be readily ascertained that nothing is dissolved chemically when water is dirty, we are only correct when we employ pure water.
94. Precautions to ensure correct Graduation.—Those who possess a boiling-point thermometer should satisfy themselves that it has been correctly graduated. To do this, it is advisable to verify it with the reading of a standard barometer reduced to 32° F. The table of “Vapour Tension” (given atp. 62) will furnish the means of comparison. Thus, if the reduced reading of the barometer, corrected also for latitude, be 29·922, the thermometer should show 212° as the boiling-point of water at the same time and place; if 29·745, the thermometer should read 211·7; and so on as per table. In this way the error of the chief point of the scale can be obtained. Other parts of the scale may be checked with a standard thermometer, by subjecting both to the same temperature, and comparing their indications. The graduations as fixed by some makers are not always to be trusted; and this essential test should be conducted with the utmost nicety and care.
Admiral FitzRoy writes, in hisNotes on Meteorology:—“Each degree of the boiling-point thermometer is equivalent to about 550feet of ascent, or one-tenth to55 feet; therefore, the smallest error in the graduation of the thermometer itself will affect the height deduced materially.
“In the thermometer which is graduated from 212° (the boiling-point) to 180°, similarly to those intended for the purpose of measuring heights, there must have been a starting point, or zero, from which to begin the graduation. I have asked an optician in London how he fixed that zero, the boiling-point. ‘By boiling water at my house,’ he replied. ‘Where is your house?’ In such a part of the town, he answered. I said: ‘What height is it above the sea?’ to which he replied, ‘I do not know;’ and when I asked the state of the barometer when he boiled the water, whether the mercury was high or low, he said that he had not looked at it! Now, as this instrument is intended to measure heights and to decide differences of some hundred, if not thousand feet upwards, at least one should endeavour to ascertain a reliable starting point. From inquiries made, I believe that the determination of the boiling-point of ordinary thermometers has been very vague, not only from the extreme difficulties of the process itself (which are well known to opticians), but from the radical errors of not allowing for the pressure of the atmosphere at the time of graduation—which may be much, even an inch higher or lower, than the mean, or anygiven height—while the elevation of the place above the level of the sea is also unnoticed. Then there is another source of error, a minor one, perhaps: the inner limit, the 180° point, is fixed only by comparison with another thermometer; it may be right, or it may be very much out, as may be the intermediate divisions; for the difficulty of ascertaining degree by degree is great: and it must be remembered that the measurement of a very high mountain depends upon those inner degrees from 200° down to 180°, thereabouts. Hence, the difficulty of making a reliable observation by boiling water seems to be greater than has been generally admitted.”
95. Method of Calculating Heights from Observations with the Mountain Thermometer.—Having considered how to make observations with the proper care and accuracy, it becomes necessary to know how to deduce the height by calculation. That a constant intimate relation exists between the boiling temperature of water and the pressure of the air, we have already learned. This knowledge is the result of elaborate experiments made by several scientific experimentalists, who have likewise constructed formulæ and tables for the conversion of the boiling temperatures into the corresponding pressures of vapour, or, which is equivalent, of the atmosphere, when the operation is performed in the open air. As might be expected, there is not a perfect accord in the results arrived at by different persons. Regnault is the most recent, and his experiments are considered the most reliable.
From Regnault’s table of vapour tension, we can obtain the pressure in inches of mercury at 32°, which corresponds to the observed boiling-point; orvice versa, if required. From the pressure, the height may be deduced by the method for finding heights by means of the barometer.
The following table expresses very nearly the elevation in feet corresponding to a fall of 1° in the temperature of boiling water:—
These numbers agree very well with the results of theory and actual observation. The assumption is that the boiling-point will be diminished 1° for each 520 feet of ascent until the temperature becomes 210°, then 530 feet of elevation will lower it one degree until the water boils at 200°, and so on; the air being at 32°.
LetHrepresent the vertical height in feet between two stations;Bandb, the boiling-points of water at the lower and upper stations respectively;f, the factor found in the above table. Then
H=f(B-b)
Further, letmbe the mean temperature of the stratum of air between the stations. Now, if the mean temperature is less than 32°, the column of air will be shorter; and if greater, longer than at 32°. According to Regnault, air expands1⁄491·13or ·002036 of its volume at 32°, for each degree increase of heat. Calling the correction due to the mean temperature of airC, its value will be found from the equation,
C=H(m- 32) ·002036
Calling the corrected heightH′, it will be found from the formula,
H′=H+H(m- 32) ·002036
that is,
H′=H{1 + (m- 32) ·002036}
and substituting the value ofH,
H′=f(B-b) {1 + (m- 32) ·002036}
Strictly, according to theoretical considerations, there is a correction due to latitude, as in the determination of heights by the barometer; but its value is so small that it is practically of no importance.
If a barometer be observed at one of the stations, the table of vapour tensions (p. 62) will be useful in converting the pressure into the corresponding boiling-point, orvice versa; so that the difference of height may be found either by the methods employed for the boiling-point thermometer or the barometer.
In conclusion, it may be remarked that observers who have good instruments at considerable elevations, as sites on mountains or plateaus, would confer a benefit to science, by registering for a length of time the barometer along with the boiling temperature of water, as accurately as possible. Such observations would serve toverify the accuracy of theoretical deductions, and fix with certainty the theoretical scale with the barometer indications.
Example, in calculating Heights from the Observations of the Boiling-point of Water.—1. At Geneva the observed boiling-point of water was 209°·335; on the Great St. Bernard it was 197°·64; the mean temperature of the intermediate air was 63°·5; required the height of the Great St. Bernard above Geneva.
Method by formula:—
H′=f(B-b) {1 + (m- 32°) ·002036}
In this casefis between 530 and 550, or 540.
Method by Tables supplied with boiling-point apparatus made by Messrs. Negretti and Zambra:—
96. Thermometers for Engineers.—1st. Salinometer.—Under the circumstances at which fresh water boils at 212°, sea water boils at 213°·2. The boiling temperature is raised by the chemical solution of any substance in the water, and the more with the increase of matter dissolved.
From a knowledge of this principle, marine engineers make use of the thermometer to determine the amount of salts held in solution by the water in the boilers of sea-going steamers. Common sea-water contains1⁄33of its volume of salt and other earthy matters. As evaporation proceeds, the solution becomes proportionally stronger, and more heat is required to produce steam. The following table from the work of Messrs. Main and Brown, on the Marine Steam-Engine, shows the relation between the boiling-point under the mean pressure of the atmosphere, or 80 inches of mercury, and the proportion of matter dissolved in the water:—
Fig. 74.
When the salts in solution amount to12⁄33, the water is saturated. It has also been ascertained that, when a solution of4⁄33is attained, incrustation of the substances commences on the boiler. Hence, it is a rule with engineers to expel some of the boiling water, when the thermometer indicates a temperature of 216°, and introduce some more cold water, in order to prevent incrustation, which not only injures the boiler, but opposes the passage of heat to the water. The thermometer used for this purpose should be very accurately graduated, and the scale must be considerably higher than, though it need not read much below 212°.
2nd. Pressure Gauge.—The elasticity of gases augments by increase of temperature, andvice versa; it follows, therefore, that when steam is generated in a closed boiler, its temperature rises beyond the boiling temperature of 212°, owing to the increased pressure upon the water. The law connecting the pressure and the corresponding temperature of steam is the same as that upon which the boiling of fluids under diminished atmospheric pressure takes place. Hence, the indications of the thermometer become exponents of steam pressure. Engineers are furnished, in works on the steam-engine, with tables, from which the pressure corresponding to a given temperature, or the converse, can be obtained by mere inspection.
Fig. 74 represents the thermometer employed as a steam-pressure gauge. It is fitted in a brass case, with screw-plug and washers for closing the boiler when the thermometer is not in use. The scale shows the pressure corresponding to the temperature, from 15 to 120 lbs., above the atmospheric pressure, which is usually taken as 15 lbs. on the square inch.
INSTRUMENTS FOR ASCERTAINING THE HUMIDITY OF THE AIR.
97. Hygrometric Substances.—The instruments devised for the purpose of ascertaining the humidity of the atmosphere are termedhygrometers. The earliest invented hygrometers were constructed of substances readily acted upon by the vapour in the air, such as hair, grass, seaweed, catgut, &c., which all absorb moisture, and thereby increase in length, and when deprived of it by drying they contract. Toy-like hygrometers, upon the principle of absorption, are still common as ornaments for mantel-pieces. A useful little instrument of this class, formed from the beard of the wild oat, is made to resemble a watch in external appearance, and is designed to prove the dampness or dryness of beds: a moveable hand points out on the dial the hygrometric condition of the clothes upon which the instrument is laid.
Fig. 75.
98. Saussure’s Hygrometer, formerly used as a meteorologic instrument, but now regarded as an ornamental curiosity, is represented in fig. 75. Its action depends upon a prepared hair, fixed at one end to the frame of the instrument, and wound round a pulley at the other. The pulley carries a pointer which has a counterpoise sufficient to keep the hair stretched. By this means the shrinking and lengthening of the hair cause the pointer to traverse a graduated arc indicating the relative humidity.
Such instruments, however ingenious, are not of scientific value; because they do not admit of rigid comparison, are liable to alter in their contractile and expansive properties, and cannot be made to indicate precisely alike.
99. Dew-Point.—The amount of water which the air can sustain in an invisible form increases with the temperature; but for every definite temperature there is a limit to the amount of vapour which can be thus diffused. When the air is cooled, the vapour present may be more than it can sustain; part will then be condensed as dew, rain, hail or snow, according to the meteorologic circumstances. The temperature which the air has when it is so fully saturated with vapour that any excess will be deposited as dew, is called thedew-point.
100. Drosometer.—“To measure the quantity of dew deposited each night, an instrument is used called aDrosometer. The most simple process consists in exposing to the open air bodies whose exact weight is known, and then weighing them afresh after they are covered with dew. According to Wells, locks of wool, weighing about eight grains, are to be preferred, which are to be divided [formed] into spherical masses of the diameter of about two inches.”—Kœmtz.
101. Humidity.—The proportion existing between the amount of vapour actually present in the air at any time, and the quantity necessary to completely saturate it, is calledthe degree of humidity. It is usually expressed in a centesimal scale, 0 being perfect dryness, and 100 complete saturation.
The pressure, or tension, of vapour at the dew-point temperature, divided by the tension of vapour at the air temperature and the quotient multiplied by 100, gives the degree of humidity. (Regnault’s Tables should be used.)
Hence the utility of instruments for determining the dew-point.