as well as thickness; and, as one end communicates with this
wheel-work, whilst the other end is fixed and immoveable, no sooner does
it begin to dilate than it presses against the wheel-work, and sets in
motion the index, which points out the degrees of dilatation on the
dial-plate.

[Illustration: Plate I. Vol. I. p. 38.

Fig. 1. Pyrometer.
A.A Bar of Metal.
1.2.3 Lamps burning.
B.B Wheel work.
C Index.

Fig. 2
A.A Glass tubes with bulbs.
B.B Glasses of water in which they are immersed.]

EMILY.

This is, indeed, a very curious instrument; but I do not understand the
use of the wheels: would it not be more simple, and answer the purpose
equally well, if the bar, in dilating, pressed against the index, and
put it in motion without the intervention of the wheels?

MRS. B.

The use of the wheels is merely to multiply the motion, and therefore
render the effect of the caloric more obvious; for if the index moved no
more than the bar increased in length, its motion would scarcely be
perceptible; but by means of the wheels it moves in a much greater
proportion, which therefore renders the variations far more conspicuous.

By submitting different bodies to the test of the pyrometer, it is found
that they are far from dilating in the same proportion. Different metals
expand in different degrees, and other kinds of solid bodies vary still
more in this respect. But this different susceptibility of dilatation is
still more remarkable in fluids than in solid bodies, as I shall show
you. I have here two glass tubes, terminated at one end by large bulbs.
We shall fill the bulbs, the one with spirit of wine, the other with
water. I have coloured both liquids, in order that the effect may be
more conspicuous. The spirit of wine, you see, dilates by the warmth of
my hand as I hold the bulb.

EMILY.

It certainly does, for I see it is rising into the tube. But water, it
seems, is not so easily affected by heat; for scarcely any change is
produced on it by the warmth of the hand.

MRS. B.

True; we shall now plunge the bulbs into hot water, (PLATE I. Fig. 2.)
and you will see both liquids rise in the tubes; but the spirit of wine
will ascend highest.

CAROLINE.

How rapidly it expands! Now it has nearly reached the top of the tube,
though the water has hardly begun to rise.

EMILY.

The water now begins to dilate. Are not these glass tubes, with liquids
rising within them, very like thermomet