A Treatise on Meteorological Instruments Part 12

CHAPTER IX.

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 gla.s.s 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 princ.i.p.al 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 r.e.t.a.r.d 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only portion of the instrument likely to be affected by pressure, has been, in Negretti and Zambra's new instrument, superseded by a strong outer cylinder of gla.s.s, containing mercury and rarefied air; by this means the portion of the instrument susceptible of compression, has been so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.

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 s.h.i.+eld, 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 pa.s.s 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 bra.s.s 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.

[Ill.u.s.tration: Fig. 69.]

=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 bra.s.s and steel, the specific gravity of these metals being 839 and 781 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 106. 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 bra.s.s and steel, to form compound bars of thin bars of these metals riveted together; and which will be found to a.s.sume a slight curve in one direction when heat has expanded the bra.s.s more than the steel, and a slight one in the contrary direction when cold has contracted the bra.s.s more than the steel.

"The indications of the instrument record the motions under changes of temperature of such compound bars; in which the proportion of bra.s.s, the more dilatable metal, is two-thirds, and of steel one-third.

[Ill.u.s.tration: Fig. 70.]

"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, at _e_, 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."

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.

[Ill.u.s.tration: Fig. 71.]

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 the _Library 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 1/4 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 at _e 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 a _register 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.

CHAPTER X.

BOILING-POINT THERMOMETERS.

=91. Ebullition.=--The temperature at which a fluid _boils_ is called the _boiling-point_ of that particular fluid. It is different for different liquids; and, moreover, in the same liquid it varies with certain changes of circ.u.mstance. 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 lat.i.tude of London, by a column of 29905 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 subst.i.tute 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 a.s.sert 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.

Messrs. Negretti and Zambra's arrangement of the instrument is shown in figures 72 and 73.

[Ill.u.s.tration: Fig. 72.]

[Ill.u.s.tration: Fig. 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 by _D 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 low down, _G_, to give vent to the steam. As the steam is generated, it rises in the inner tube, pa.s.ses down between the tubes, and flows away at _G_. The thermometer is pa.s.sed 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 from _G_; 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 at p. 62) will furnish the means of comparison. Thus, if the reduced reading of the barometer, corrected also for lat.i.tude, be 29922, the thermometer should show 212 as the boiling-point of water at the same time and place; if 29745, the thermometer should read 2117; 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 his _Notes on Meteorology_:--"Each degree of the boiling-point thermometer is equivalent to about 550 _feet of ascent_, or one-tenth to 55 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 any _given 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 formulae 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; or _vice 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:--

Boiling Temperatures Elevation in Feet between. for each Degree.

214 and 210-- 520 210 and 200-- 530 200 and 190 550 190 and 180 570

These numbers agree very well with the results of theory and actual observation. The a.s.sumption 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.

Let _H_ represent the vertical height in feet between two stations; _B_ and _b_, 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_)