mark off divisions of the same extent, in both directions, to the extremities of the tube, the instrument will be completly graduated.

In the centigrade thermometer, the freezing point is Co, and the boiling point 100°; in Reaumur's, the freezing point is marked 0°, and the boiling point 80°; and in Fahrenheit's, which is the thermometer in common use in this country, the freezing point is marked 32°, and the boiling point 212°. Assuming, then, the boiling point to have been determined under the same pressure for all three, and consequently to indicate the same absolute temperature, we have

if c, R, and F,° represent respectively the degrees of the centigrade thermometer of Reaumur's, and of Fahrenheit's, denoting the same absolute temperature.

The mercurial thermometer was constructed under the idea, that equal differences of temperature would cause the bulk of a body to vary by equal differences. Now, except in the case of gases, this assumption is not borne out by the facts of the case; but by a fortunate coinci dence, the increasing rate of the expansion of the mercury is exactly compensated by the comparatively small expansion of the glass, so that the indications of the temperature given by the ordinary mercurial thermometer are strictly accurate. The mere fact of speaking of the indications of a thermometer as accurate implies, that there is some mode of testing this accuracy, and, consequently, some idea of comparing temperatures independently of the observation of the variation in the bulk of a body, produced by a difference of temperature; and, in fact, the comparison of temperatures depends on the following considerations :

1o. Two bodies are said to have the same temperature, if when placed in contact the temperature of either remains unaltered by the other.

2o. When bodies of different temperature are placed in contact, the temperature of the hotter body decreases, and that of the colder increases, till both become of the same temperature.

30. If the bodies thus placed in contact be both of the same material, and be equal to each other in weight, the in

H

crease of temperature in one will be equal to its decrease in the other, and the product of the whole mass into the temperature will remain the same.

In order to test the accuracy of the common thermometer, Dr. Brooke Taylor heated two equal weights of water, till their temperatures, as indicated by the thermometer, were respectively 100° and 200°; and on mingling these bodies of water together, the thermometer indicated correctly 150°, as the temperature of the mixture.

THE BAROMETER.

There are various forms of the barometer, to suit the various circumstances under which it is to be used; but in all of them there is a straight tube, about 33 inches long, closed at one end, for holding the column of mercury, of which the altitude is to be observed. This is called the barometer tube, and, being filled with mercury, the open end of it is placed in communication either with an open cistern containing mer cury, or with a bent tube containing mercury, which is exposed to the action of the atmosphere.

Fig. 1. represents a form of barometer, called the standard. barometer, adapted for use in fixed observatories. The bore of the barometer tube, A c, should not be less than one-fourth

of an inch diameter, and if it be one-third of an inch it is better, in order to avoid any considerable error from the capillary depression. The diameter of the open end, A в, of the barometer should be as large as it can conveniently be made, in order that the variation in the altitude of the mercury in a o should affect as little as possible the altitude of the surface A B. In order, however, to obtain the true altitude of the mercury within this barometer, the position of the surface A B must be taken into account, as well as that of the extremity of the mercurial column AC, the height of which above an assumed zero point is given by the scale attached to the barometer. This is done by making the scale movable, and terminating in an ivory point: when this point and its reflection appear to touch one another, the height indicated is correct.

Fig. 1.

Tubes of small diameters require correction for capillarity. The mercury is depressed in the glass tube from the repul

sion between the two substances, and the following corrections must be added :

The syphon barometer, Fig. 2, gives the altitude of the mercurial column independently of errors from capillary depression; for, on account of the equal size of the bore throughout the tube, the extremities of the columns A and c are equally depressed.

In the ordinary form of the portable barometer the tube is immersed in a reservoir with a leathern bottom, and furnished with a screw f, by turning which the mercury can be forced up, and made entirely to fill the tube, to prevent it from jerking about, and injuring the instrument during its carriage from place to place. This screw can be made to regulate the surface of the mercury in the cistern, and always bring it into line with the zero of the scale. For this purpose the instrument is furnished with a gauge or float h. The gauge consists of a sall ivory cylinder, fixed to a float of cork, which rests on the top of the mercury. The upper end of this cylinder works in a groove, hollowed out of a second piece of ivory, which is fixed to the instrument, and has a fine line upon it denoting the zero o the scale the floating cylinder has a corresponding fine line cut upon it; and, before observing the reading of the instrument, the two lines must be brought into exact coincidence by turning the screw f.

Fig. 2.

Let a b, Fig. 3, be the glass tube plunged into the

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α

mercury in the cistern c, and D be the surface-line of the Auid in the cistern, made level with the commencement of the scale, in the construction of the instrument: then the extremity of the mercurial column in the tube at this time is called the neutral point. When the mercury rises in the tube, a portion equal to the rise leaves the cistern, and the surface-line falls towards the dotted line e; and, being lower than the surface from which its neutral point was measured, the indicated variation in the atmosphere is too little; but turn the screw ƒ forwards, until the lines on the float h coincide, and the mercury then records the exact change; when a depression occurs, the mercury, sinking from the tube into the cistern, raises the surface-line towards g: in this case the screw f must be turned back, until the leather at the bottom of the cistern be sufficiently loosened to allow the mercury to assume its proper level at the surface D.

b

C

Fig. 3.

When there is not a gauge to the barometer, the relative capacities of the cistern and tube are ascertained by experiment, in the construction of the instrument, and marked thereon, as is also its neutral point.

The correction to be applied to the reading of the scale, to obtain the correct height, may be computed as follows:

Let h be the neutral point altitude of the extremity of the mercurial column, when the zero point of the scale coincides with the surface D, and let h' be its apparent altitude in any other case; let also к represent the sectional area of D, and k that of the tube; so that is the capacity

K

k

of the cistern relatively to the tube, which is marked on the instrument; then the distances, through which the surfaces in the tube, and cistern, will rise and fall respectively, will be manifestly inversely proportional to the areas of their surfaces; if, therefore, dh represent the required correction, viz., the distance through which the mercury falls in the cistern, hh: dh'::K:k.

and the true altitude of the barometer is given by the equation

k

h' + dh' = k + ~ (h' — h).

K

When the indicated height, h', is above the neutral point h, h'-h is positive, and the correction dh' is to be added, to give the correct height; but when the mercury is below the neutral point, h'-h is negative, and the correction Sh' is to be subtracted, to give the correct height. Hence the following rule :

When the mercury in the tube is above the neutral point, the difference between it and the neutral point is to be divided by the capacity, and the quotient added to the observed height, to give the correct height; and, when the mercury is below the neutral point, the difference is to be divided as before, and the quotient subtracted from the observed height will give the correct height.

Ex.—Let, or the capacity of the cistern relatively to the tube, be 40, and the neutral point altitude be 30 inches.

The portable, or mountain barometer, has received great Improvements. It is now produced in a form, in which the