higher point.

We are thus furnished with the means of determining the Let the instrument be immersed in steam arising from boiling water, mark off as before the termination of the mercurial column, reading the barometer at the same moment. If the pressure of the atmosphere be 29.905 inches, this point will denote 212°; and for any other pressure the true value of the mark may be found from the above table.

In performing this operation it will be found most convenient to employ Regnault's apparatus (see page 12). The arrangement will best be perceived from Fig. 2, which represents the interior of the instrument.

A is a thermometer having its bulb a little above E, the level of the boiling water. The course which the steam is forced to take is denoted by the arrow-heads. It is thus seen that the steam must pass up along the thermometer tube and down again, until finally it leaves the apparatus by the orifice C. The whole of the tube of the instrument is thus thoroughly surrounded by steam, and by a cylinder of the temperature of the steam. D is a bent glass tube, open to the atmosphere, and containing a little water, which shews by a difference of level in the two limbs if the pressure of the steam in the interior is greater than that of the atmosphere without. It has been ascertained that, if the orifice

is sufficiently wide, this difference is too small to affect the temperature of the thermometer, and thus the gauge D may be dispensed with.

The thermometer is inserted through a closely fitting slit in a thick piece of india-rubber which rests upon the top of the apparatus, and the stem is lowered until the column of mercury just appears above the india-rubber; and thus nearly all the column as well as the bulb is exposed to the vapour of boiling water. This apparatus is generally formed of copper, and distilled water should if possible be used. It ought also to be noted that in marking off the

points the lower or freezing point should always be determined first; the reason for this will be afterwards given.

20. Graduation. Fahrenheit's scale. The relative diameter of the bore at different parts of the tube having now been determined by calibration (Art. 17), and the two fixed points marked, it is easy to graduate the instrument. If the scale is to be that of Fahrenheit, the lower point is called 32° and the upper point (provided the atmospheric pressure be 29.905 inches) 212°. There are thus 180 divisions between the two points, and these are marked upon the tube in the following manner. The whole instrument is covered over with wax sufficiently thin to allow the two marks (previously blackened) to be visible, and a needle. attached to a dividing engine scratches the graduations in the wax. The thermometer is then exposed to hydrofluoric acid or its vapour, which attacks the glass where the wax has been scratched off. The length of each degree is regulated by the previously determined diameter of the bore in such a manner that the internal capacity of the tube between the two marks is divided into 180 equal parts. The graduation is generally extended below the freezing point, and sometimes above the boiling point. In this thermometer a temperature 32° below the freezing point is termed zero, while one ten degrees lower is called minus 10°, or 10° below zero, and so on.

21. Other scales. Besides Fahrenheit's scale there are two others, those of Celsius and Reaumur. The former of these is also called Centigrade, and is used throughout France, and the latter very generally in Germany.

In the Centigrade thermometer the freezing point of water is termed c°, or zero, while the boiling point is reckoned equal to 100. A degree Centigrade is therefore greater than a degree Fahrenheit in the proportion of nine to five. The boiling point in this thermometer, or 100°, is defined to be the temperature of steam under the barometric pressure

of 760 millimètres, or 29.922 inches of mercury reduced to the freezing point of water at the latitude of Paris. This is slightly different from the corresponding point, or 212°, in Fahrenheit's scale, which, as we have seen, denotes the temperature of steam under the pressure of 29.905 inches of mercury reduced to 32° at the latitude of London. It must, however, be borne in mind that the force of gravity, and therefore the absolute pressure towards the earth of the same mass of matter, is somewhat greater in London than at Paris, so that 29.922 inches of mercury in Paris are equal in absolute pressure to 29.914 inches at London (see Art. 143). This still leaves a slight difference between the absolute pressure of the steam in the two cases, and hence the upper points of these two instruments will not quite correspond in temperature; but this difference is so very small that for ordinary purposes it may be neglected. To reduce Fahrenheit to Centigrade the following formula is made use of, C

=

(F

5

32); while to reduce Centigrade

Thus, were it required to find what degree Centigrade corresponds to 77° Fahr., we should proceed in the following manner. Subtracting 32° from 77° we find that this temperature is 45° above the freezing point, or Centigrade zero, and taking of 45°, since Centigrade degrees are greater than those of Fahrenheit in this proportion, we find that 25° Cent. corresponds to 77° Fahr.

Again, were it required to find what degree Centigrade corresponds to 38°.2 Fahr., we should have, as above

In the scale of Reaumur the distance between the freezing

and boiling point is divided into 80 parts, and the boiling

point is defined to be the temperature of steam under the pressure of 760 millimètres of mercury, the force of gravity being that which corresponds to latitude 45°.

To reduce Fahrenheit to Reaumur we have therefore

4

the following expression, R = (F-32); while to reduce

Reaumur to Fahrenheit we have F

=

9 R

4

+32.

22. Correction for change of zero. It is found that thermometers are liable to an alteration of their zero points, especially when the bulb has been filled not long before graduation. This displacement is of the following nature. Immediately after graduation 32° will of course denote the temperature of melting ice, but when some time has elapsed a thermometer placed in melting ice will no longer give this reading, but one somewhat higher, perhaps 32.4 or 32.5.

When an instrument has been graduated shortly after the filling of the bulb, this displacement may in the course of years amount to nearly 2° Fahr., but it is believed that this is the extreme limit of the change. But if the bulb has been kept for some time before graduation, and has also been well annealed, the change is much less nevertheless it may possibly amount in the course of years to six or seven tenths of a degree. Besides this progressive and permanent change there is also a temporary one, produced by heating and suddenly cooling the instrument. For instance, if a thermometer have first of all its freezing point determined by melting ice, if it then be plunged into boiling water, then suddenly withdrawn, and finally plunged again into ice, the freezing point will be found to have changed—the instrument may now read 31.8, and it will not recover its true reading until ten days or a fortnight have elapsed. This is the reason why the freezing point is always marked off first in constructing the instrument.