A Treatise on Meteorological Instruments Part 7

=49. Small Size Aneroids.=--The patent for the Aneroid having expired, Admiral FitzRoy urged upon Messrs. Negretti & Zambra the desirability of reducing the size at which it had hitherto been made, as well as of improving its mechanical arrangement, and compensation for temperature.

They accordingly engaged skilful workmen, who, under their directions, and at their expense, by a great amount of labour and experiment, succeeded in reducing its dimensions to two inches in diameter, and an inch and a quarter thick. The exact size and appearance of this aneroid are shown in fig. 34. The compensation is carefully adjusted, and the graduations of the dial ascertained under reduced pressure, so that they are not quite equal, but more accurate.

=50. Watch Aneroid.=--Subsequently the aneroid has been further reduced in size and it can now be had from an inch and a quarter to six inches in diameter. The smallest size can be enclosed in watch cases, fig. 35, or otherwise, so as to be adapted to the pocket. By a beautifully simple contrivance, a milled rim is adjusted to move round with hand pressure, and carry a fine index or pointer, outside and around the scale engraved on the dial, or face, for the purpose of marking the reading, so that the subsequent increase or decrease of pressure may be readily seen. These very small instruments are found to act quite as correctly as the largest, and are much more serviceable. Besides serving the purpose of a weather-gla.s.s in the house or away from home, if carried in the pocket, they are admirably suited to the exigencies of tourists and travellers.

They may be had with scale sufficient to measure heights not exceeding 8,000 feet; with a scale of elevation in feet, as well as of pressure in inches, engraved on the dial. The scale of elevation, which is for the temperature of 50, was computed by Professor Airy, the Astronomer Royal, who kindly presented it to Messrs. Negretti and Zambra, at the same time suggesting its application. Moderate-sized aneroids, fitted in leathern sling cases, are also good travelling instruments, and will be found serviceable to pilots, fishermen, and for use in coasting and small vessels, where a mercurial barometer cannot be employed, because requiring too much s.p.a.ce.

[Ill.u.s.tration: Fig. 35.]

Admiral FitzRoy, in a communication to the _Mercantile Marine Magazine_, December, 1860, says:--"Aneroids are now made more portable, so that a pilot or chief boatman may carry one in his pocket, as a railway guard carries his timekeeper; and, thus provided, pilots cruising for expected ships would be able to caution strangers arriving, if bad weather were impending, or give warning to coasters or fishing boats. Harbours of Refuge, however excellent and important, are not always accessible, even when most wanted, as in snow, rain, or darkness, when neither land, nor buoy, nor even a lighthouse-light can be seen."

=51. Measurement of Heights by the Aneroid.=--For measuring heights not exceeding many hundred feet above the sea-level by means of the aneroid, the following simple method will suffice:--

Divide the difference between the aneroid readings at the lower and upper stations by 0011; the quotient will give the approximate height in feet.

Thus, supposing the aneroid to read at the

Lower Station 30385 inches.

Upper Station 30025 ------ Difference 360 ======

Divided gives 360/0011 = 327 feet.

As an ill.u.s.tration of the mode in which the aneroid should be used in measuring heights, the following example is given:--

A gentleman who ascended Helvellyn, August 12th, 1862, recorded the following observations with a pocket aneroid by Negretti and Zambra:--

Near 10 a.m., at the first milestone from Ambleside, found by survey to be 188 feet above the sea, the aneroid read 2989 inches; about 1 p.m., at the summit of Helvellyn, 2681; and at 5 p.m., at the milestone again, 2976. The temperature of the lower air was 57, of the upper, 54. Hence the height of the mountain is deduced as follows:--

Inches.

Reading at 10 a.m. 2989 " 5 p.m. 2976 ------ Mean 29825 Table I.[5] 1010 Upper Reading 2681 " 3796 ----- Difference 2786 Mean Temperature 555, gives in Table II. 1048 ----- 2920 Lat. 55 N., gives in Table III. 9991 ----- 2917 Table IV. 5 ----- Difference of height 2922 Height of lower station 188 ----- " Helvellyn 3110

In Sir J. Hersch.e.l.l's _Physical Geography_ it is given as 3115 ft.

So near an agreement is attributable to the excellence of the aneroid, and the careful accuracy of the observer.

52. METALLIC BAROMETER.

This instrument, the invention of M. Bourdon, has a great resemblance to the aneroid, but is much simpler in arrangement. The inventor has applied the same principle to the construction of metallic steam-pressure gauges.

We are here, however, only concerned with it as constructed to indicate atmospheric pressure. It consists of a long slender flattened metallic tube, partially exhausted of air, and hermetically closed at each end, then fixed upon its centre, and bent round so as to make the ends face each other. The transverse section of this tube is an elongated ellipse.

The principle of action is this: interior pressure tends to straighten the tube, external pressure causes it to coil more. Hence as the atmospheric pressure decreases, the ends of the tube become more apart.

This movement is augmented and transferred by a mechanical arrangement of small metallic levers to a radius bar, which carries a rack formed on the arc of its circle. This moves a pinion, upon the arbour of which a light pointer, or "hand," is poised, which indicates the pressure upon a dial.

When the pressure increases, the ends of the tube approach each other, and the pointer moves from left to right over the dial. The whole mechanism is fixed in a bra.s.s case, having a hole at the back for adjusting the instrument to the mercurial barometer by means of a key, which sets the pointer without affecting the levers. The dial is generally open to show the mechanism, and is protected by a gla.s.s, to which is fitted a moveable index.

This barometer is very sensitive, and has the advantage of occupying little s.p.a.ce, although it has not yet been made so small as the aneroid.

Both these instruments admit of a great variety of mounts to render them ornamental. The metallic barometer can be constructed with a small clock in its centre, so as to form a novel and beautiful drawing-room ornament.

Admiral FitzRoy writes, "Metallic barometers, by Bourdon, have not yet been tested in very moist, hot, or cold air for a sufficient time. They are dependent, or secondary instruments, and liable to deterioration. For limited employment, when sufficiently compared, they may be very useful, especially in a few cases of electrical changes, _not foretold or shown by mercury_, which these seem to indicate remarkably."

They are not so well adapted for travellers, nor for measurements of considerable elevations, as aneroids.

CHAPTER VI.

INSTRUMENTS FOR ASCERTAINING TEMPERATURE.

=53. Temperature= is the energy with which heat affects our sensation of feeling.

Bodies are said to possess the same temperature, when the amounts of heat which they respectively contain act outwardly with the same intensity of transfer or absorption, producing in the one case the sensation of warmth, in the other that of coldness. Instruments used for the determination and estimation of temperatures are called _Thermometers_.

Experience proves that the same body always occupies the same s.p.a.ce at the same temperature; and that for every increase or decrease of its temperature, it undergoes a definite dilatation or contraction of its volume. Provided, then, a body suffers no loss of substance or peculiar change of its const.i.tuent elements or atoms, while manifesting changes of temperature it will likewise exhibit alterations in volume; the latter may, therefore, be taken as exponents of the former. The expansion and contraction of bodies are adopted as arbitrary measures of changes of temperature; and any substance will serve for a thermometer in which these changes of volume are sensible, and can be rendered measureable.

=54. Thermometric Substances.=--Thermometers for meteorological and domestic purposes are constructed with liquids, and generally either mercury or alcohol, because their alterations of volume for the same change of temperature are greater than those of solids; while being more manageable, they are preferred to gases. Mercury is of all substances the best adapted for thermometric purposes, as it maintains the liquid state through a great alteration of heat, has a more equable co-efficient of expansion than any other fluid, and is peculiarly sensitive to changes of temperature. The temperature of solidification of mercury, according to Fahrenheit's scale of temperature, is -40; and its temperature of ebullition is about 600. Sulphuric ether, nitric acid, oil of sa.s.safras, and other limpid fluids, have been employed for thermometers.

=55. Description of the Thermometer.=--The ordinary thermometer consists of a gla.s.s tube of very fine bore, having a bulb of thin gla.s.s at one extremity, and closed at the other. The bulb and part of the tube contains mercury; the rest of the tube is a vacuum, and affords s.p.a.ce for the expansion of the liquid. This arrangement renders very perceptible the alterations in volume of the mercury due to changes of temperature. It is true, the gla.s.s expands and contracts also; but only by about one-twentieth of the extent of the mercury. Regarding the bulb, then, as unalterable in size, all the changes in the bulk of the fluid must take place in the tube, and be exhibited by the expansion and contraction of the column, which variations are made to measure changes of temperature.

56. STANDARD THERMOMETER.

The peculiarities in the construction of thermometers will be best understood by describing the manufacture of a _Standard Thermometer_, which is one of the most accurate make, and the scale of which is divided independently of any comparison with another thermometer. Fig. 36 is an ill.u.s.tration of such an instrument, on a silvered bra.s.s scale.

[Ill.u.s.tration: Fig. 36]

_Selection of Tube._--In selecting the gla.s.s tube, much care is requisite to ascertain that its bore is perfectly uniform throughout. As received from the gla.s.s-house, the tubes are generally, in their interior, portions of very elongated cones, so that the bore is wider at one end than at the other. With due care, however, a proper length of tube can be selected, in which there is no appreciable difference of bore. This is ascertained by introducing into the tube a length of mercury of about a half or a third of an inch, and accurately measuring it in various positions in the tube.

To accomplish this, the workman blows a bulb at one end of the tube, and heats the bulb a little to drive out some of the air. Then, placing the open end in mercury, upon cooling the elasticity of the enclosed air diminishes, and the superior pressure of the atmosphere drives in some mercury. The workman stops the process so soon as he judges sufficient mercury has entered. By cooling or heating the bulb, as necessary, the mercury is made to pa.s.s from one end of the tube to the other. Should the length of this portion of mercury alter in various parts of the bore, the tube must be rejected. If it is, as nearly as possible, one uniform length, the tube is set aside for filling.

The _bulb_ is never blown by the breath, but by an elastic caoutchouc ball containing air, so that the introduction of moisture is avoided. The spherical form is to be preferred; for it is best adapted to resist the varying pressure of the atmosphere. The bulbs should not be too large, or the mercury will take some time to indicate sudden changes of temperature.

Cylindrical bulbs are sometimes desirable, as they offer larger surfaces to the mercury, and enable thermometers to be made more sensitive.

The _mercury_, with which the bulb is to be filled, should be quite pure, and freed from moisture and air by recent boiling.

_Filling the Tube._--The filling is effected by heating the bulb with the flame of a spirit-lamp, while the open end is embedded in mercury. Upon allowing the bulb to cool, the atmospheric pressure drives some mercury into it; and the process of heating and cooling is thus continued until sufficient mercury is introduced. The mercury is next boiled in the tube, to expel any air or moisture that may be present. In order to close the tube and exclude all air, the artist ascertains that the tube contains the requisite quant.i.ty of mercury; then, by holding the bulb over the spirit flame, he causes the mercury to fill the whole of the tube, and dexterously removing it from the source of heat, he, at the same instant, closes it with the flame of a blow-pipe. If any air remain in the tube, it is easily detected; for if the instrument be inverted, the mercury will fall to the extremity of the tube, if there is a perfect vacuum, unless the tube be so finely capillary that its attraction for the mercury is sufficient to overcome the force of gravity, in which case the mercury will retain its position in every situation of the instrument. If, however, the mercury fall and does not reach quite to the extremity of the bore, some air is present, which must be removed.

_The Graduation._--The thermometer is now prepared for graduation, the first part of which process is the determination of two fixed points.

These are given by the temperatures of melting ice and of the vapour of boiling water. Melting ice has always the same temperature in every place and under all circ.u.mstances; provided only that the water from which the ice is congealed is free from salts. The temperature of the vapour of boiling water depends upon the pressure of the atmosphere, but is always constant for the same pressure.

The fixed point corresponding to the temperature of melting ice is called the _freezing point_. It is obtained by keeping the bulb and the part of the tube occupied by mercury immersed in melting ice, until the mercury contracts to a certain point, where it remains stationary. This position of the end of the mercury is then marked upon the tube.

The _boiling point_ is not so easily determined, for the barometer must be consulted about the same time. The boiling apparatus is generally constructed of copper. It consists of a cylindrical boiler, heated from the base by a spirit lamp or charcoal fire. An open tube two or three inches in diameter and of suitable length enters the top of the boiler.

This tube is enveloped by another fixed to the top of the boiler but not opening into it, and so that the two tubes are about an inch apart. The object of the outer tube is to protect the inner tube from the cold temperature of the air. The outer tube has an opening at the top for the admission of the thermometer, and a hole near the bottom for the escape of steam through a spout. When the water is made to boil, the steam rises in the inner tube, fills the s.p.a.ce between the tubes, and escapes at the spout. The thermometer is then pa.s.sed down into the inner cylinder, and held securely from the top by means of a piece of caoutchouc. The tubes or cylinders should be of sufficient length to prevent the thermometer entering the water. This is necessary because the temperature of boiling water is influenced by any substance which it holds in chemical solution; and, moreover, its temperature increases with the depth, owing to the pressure of the upper stratum. The thermometer being thus surrounded with steam, the mercury rises in the tube. As it does so, the tube should be depressed so as always to keep the top of the mercury just perceptible.

When the temperature of the vapour is attained, the mercury ceases to rise, and remains stationary. The position of the end of the mercury is now marked upon the tube, and the "_boiling-point_" is obtained.