they should be tested from time to time. For that purpose they may, on payment of a fee, be verified at Kew Observatory.

The Thermometer is a measurer of temperature, mercury being used for ordinary temperatures. It depends for its action on the fact that all bodies with the rise and fall of their temperatures expand and contract. It consists of a glass tube closed at the top, with a bulb at its bottom end, and having mercury placed in it. A scale of degrees is fixed to the tube. As used in mines the thermometer registers the temperature of the air, and we are able to measure the difference of temperature between the air in the downcast and upcast shafts or at any desired point in the workings.

Thermometers are graduated according to three scales, viz., Fahrenheit's, which is that commonly used in England; the Centigrade scale, which is that generally used in the scientific world; and Reaumur's scale, which is that taking its name from a French philosopher, who constructed his thermometer with alcohol of such a strength that 1,000 parts at the freezing point of water became 1,080 parts at its boiling point.

On Fahrenheit's thermometer 32° indicates the freezing point and 212° the boiling point of water, and the space between these two fixed points is divided into 180 even divisions; these even divisions are produced above and below 32° and 212°.

In the Centigrade thermometer o° indicates the freezing point and 100° the boiling point of water, the space between these two points being divided into 100 even divisions. In Reaumur's o° indicates the freezing and 80° the boiling point of water, the space between these two points being divided into 80 even divisions. It is plain therefore that—

180° Fah. = 100 Cent. = 80° Reaum.

and therefore

1° Fah. =

Cent. =

Reaum.

To transfer Fahrenheit degrees to the other scales we must first subtract 32°, in order that the number of degrees from the freezing point may be ascertained. These multiplied by ths will give the equivalent number of Centigrade, and by 4ths the equivalent number of Reaumur degrees.

To reduce Centigrade and Reaumur degrees to the Fahrenheit scale, multiply by and respectively and add 32°.

If the temperature be below the zero in any of the scales, a minus sign is placed before the number thus: -5° Fah. means 37° below freezing point. The following examples may be tested :—

It must be borne in mind that a thermometer does not give the absolute expansion of the mercury, but the difference between the expansion of the mercury and that of the glass. Mercury expands about 7 times more than glass.

Mercury, or quicksilver, is a metal having the remarkable property of being fluid at ordinary temperatures. Mercury boils at 660° Fahr., giving off vapour of specific gravity 6976, and it may therefore be distilled. It always forms one of the metals of an amalgam, and is much used in the process of amalgamation and decomposition in order to separate gold and silver from their ores. Mercury is

well adapted for use in the construction of thermometers for a certain range of temperature, as it expands at a uniform rate, when heated, from considerably below o to above 300° Fahr. In colour, it is silvery white, with a striking metallic lustre. It congeals at - 40° Fahr., and in its solid state is malleable. Liquid mercury has the specific gravity of 13:59 at 60° Fahr. It contracts greatly when it solidifies, so that the density of frozen mercury is 14. Under ordinary circumstances mercury does not alter in the air, nor, if pure, will it tarnish. It gives off vapour at all temperatures. When pure it runs in small spherical drops over smooth surfaces; when impure the drops become elongated and leave a grey stain on porcelain or glass. By distillation it is easily rendered pure. Other recommendations for the use of mercury in the construction of thermometers are its low capacity for heat, in consequence of which its temperature soon changes, and the fact that it does not adhere to the glass.

The glass tube for a thermometer has a capillary or hair-like bore, and its uniformity is tested by running about half an inch of mercury down the tube

and measuring it at different points in its descent. A uniform bore will show equal measurements of the descending mercury throughout its course along the tube. The fine glass tube is blown into a bulb at one end, and great care is necessary in the blowing. The capacity of the bulb is much greater than that of the tube, so that the expansion and contraction of the mercury in the former are rendered more perceptible in the latter. To fill the bulb, the tube is held in a slightly inclined position, a funnel of paper is fastened to the top of the tube into which is placed purified mercury. By means of a lamp the bulb is heated and the contained air expands and is driven upwards until it bubbles up through the mercury in the funnel. The lamp is then removed, the air in the tube cools and contracts, forcing some of the mercury down into the bulb. The inrushing mercury replaces the air which was driven out by the heat. The lamp is again applied to the bulb, and the process repeated a few times until the mercury reaches a convenient height in the tube when at the common temperature. The quantity of liquid introduced is generally little more than sufficient to fill the bulb at the lowest temperature which the thermometer is intended to indicate. The lamp-flame is again placed under the bulb; the heat causes the mercury column to rise to the top of the tube and ooze out; whilst so doing the tube is hermetically sealed by bringing a blowpipe flame to play upon the opening in the

Fahr. may be reached before go
crystallization begins. The file
mark on the tube indicates the
freezing point on the thermo-

Bailing Bound

The

meter. The boiling point is always constant if the barometric pressure is also constant, and is fixed by placing the thermometer in a vessel in which distilled water is made to boil, see Fig. 620. generated steam passes up the partition B, thus surrounding the thermometer placed at the top, after which it takes the direction of the arrows to an outer division in the same vessel it then descends to the orifice D, where it escapes. The steam as it surrounds the thermometer is in this way unaffected by the temperature of the air outside the generator. C is a glass tube bent like the letter U, and having a little mercury placed in the bend. It guards against a break in the

uniformity of the steam tem- Frying Font

perature. If the steam is throttled, at D, the pressure will increase and give evidence of it by driving the mercury up the tube C. The point which the mercury in the thermometer reaches when it has ceased to ascend without afterwards showing signs of fluctuation is marked off with a file as the

Fig. 622.-COMPARATIVE SCALE OF ENGLISH AND
FRENCH THERMOMETERS.

boiling point. The thermometer is then attached to a board upon which the scale is marked into arbitrary divisions. If Centigrade, see Fig. 621, the space

between the two fixed points is divided into 100 equal parts, which are produced above and below as far as required. For Fahrenheit the same space is divided into 180°, the freezing point being 32°. For Reaumur the division is into 80°, the freezing point being o° as in the Centigrade.

The experiments are made when the barometer is at 29.95 inches. A mercury thermometer cannot be graduated lower than -40° on Fahrenheit's scale, because the mercury then congeals. It may be graduated upwards to 600° Fahr., but not higher, to be trustworthy, for mercury boils at 660° Fahr.

If the vacuum in the tube of a thermometer be good, on the instrument being inverted, the mercury strikes against the top of the tube with a clear ringing sound. If, after being in use some time, it is tested by placing it in melting ice, and the mercury then does not stand at the freezing point mark, the error is

called "the displacement of zero." It arises from the curious fact that sometimes bulbs do not perfectly contract for two or three years after blowing. In the very best instruments the bulbs are kept empty for that length of time before being filled. A thick glass bulb is less likely to change than a thin one.

The three thermometer scales referred to are shown in diagram form in Fig. 622. It is drawn to scale, so that a rough comparison of the three sets of figures corresponding to a height of the mercury may be seen at a glance within certain limits.

As it is almost impossible to freeze the liquid, alcohol is used in the manufacture of thermometers to register very low temperatures. There is no limit to the downward graduation of such thermometers, but their upward range is much more limited than that of mercurial thermometers.

High temperatures are measured by pyrometers. These are instruments for measuring all gradations of temperature above those which can be directly indicated by the mercurial thermometer. Many pyrometers depend for their action upon the uniformity of the expansion and contraction of metal.

In Hobson's patent hot blast pyrometer, made by Mr. J. Casartelli, of Manchester, mercury is used, and although this is incapable of directly indicating temperatures above 600° Fahr., it is made to do so by the temperature of the hot blast being first reduced to a known degree. Fig. 623 shows the instrument. By means of

the handle E it is readily attached at the cone A to the sight-hole of the tuyere. The hot blast passes through the tube, and issues out of it at right angles facing the thermometer, and induces a current of cold air to enter at the opening B and mix with it the size of the jet tube being much smaller than the bore of the body of the instrument. This diluted current, in passing to the outlet D, acts on the bulb of the thermometer, raising its temperature in proportion to the heat of the blast. The temperature of the mixed current is indicated on the glass stem, and the side scales are graduated to mark the actual heat of the hot blast, as ascertained by Siemens' copper-ball pyrometer. The outer scales are so arranged that the one marked in Fahr. degrees is placed with the 400 opposite the 152° Fahr. on the thermometer tube. The instrument should be examined occasionally to see that the tube has not slipped. The thermometer is shown in a vertical position in the illustration, but may be turned down at right angles in a line with the handle when it is not in use.

If the column of mercury in an ordinary thermometer becomes divided in carriage, the thermometer must be inverted, and the head tapped gently on a table until the mercury flows out of the bulb to the top of the tube; it must then be shaken from side to side until the mercury re-unites and returns to the bulb in one volume.

Solids expand too little, gases too much, to be practically serviceable in the construction of thermometers for ordinary temperatures, hence the use of liquid thermometers.

The precise form which these take depends entirely upon the purpose for which they are required. If they are to be kept steadily in one position, without agitation, to register the temperature of the air in the open, in a room, or at a colliery above or below ground near the downcast shaft, they may be arranged to accompany a barometer as shown in Figs. 631, 632 and 633, or alone in common boxwood form with a scale graduated from o° to 120° or 130° Fahr. on one side of the tube, and a Centigrade scale to correspond on the other; such scales meet the requirements of range in this country.

For use in an upcast shaft a self-registering thermometer is made enclosed in a stout cylindrical copper case. It is usually graduated from 40° or 50° to 600° Fahr., the reading of a mercury thermometer being quite reliable to this point.

The case is 14 inches long by 2 inches in diameter, and is open at both ends. A hinged door is provided to close over an opening cut throughout the length of the cylinder, and this door is kept securely fastened, except when access to the interior is desired. A metal scale 14 inches long is fixed internally in the centre of the cylinder. The scale is graduated sometimes from 50° to 550° Fahr., and sometimes from 40° to 600° Fahr., the divisions being cut every 2° Fahr. The glass tube containing the mercury is 12 inches in length by of an inch in external diameter, and is inserted in the centre of the scale by means of small brackets within the case. Immediately above the bulb the bore of the tube is reduced in size, and then gradually increases till it attains its full diameter, which it afterwards maintains throughout the tube. The diameter of the bore nowhere exceeds that of an ordinary sized sewing needle. At the point where the full-sized bore begins the mercury is broken into two detached portions of column by the intervention of an air speck which is about th of an inch in height when the space separating the mercury is at its minimum. When the instrument is to be used, a shake is given it by tapping the bottom of the copper cylinder against the palm of the hand, or otherwise so as not to cause injury, and yet ensure the falling of the top portion of mercury into its normal position close to the lower column. The registered temperature recorded by it, which may be that of the surface or of some point below ground, is then read and the result entered in a book, or if below 40° Fahr., another thermometer is used

C.M.H.

SS