CONVERSATION XXIII.

Of the Pyrometer and Hygrometer.

FATHER. To make our description of philosophical instruments more perfect, I shall to-day show you the construction and uses of the pyrometer and hygrometer, and conclude, to-morrow, with an account of the rain-gauge, and some directions for judging of the weather.

Emma. What do you mean by a pyrometer?

Father. It is a Greek word, and signifies a fire-measurer. The pyrometer is a machine for measuring

the expansion of solid substances, particularly metals, by heat. This instrument (Plate III, Fig. 30) will render the smallest expansions sensible to the naked eye.

Charles. Is all this apparatus necessary for the purpose?

Father. This, as far as I know, is one of the most simple pyrometers, and, admitting of an easy explanation, I have chosen it in preference to a more complicated instrument, which might be susceptible of greater nicety.

To a flat piece of mahogany, A A, are fixed three studs, B, C, and D, and at there is an adjusting screw P. HF is an index, turning very easy on the pivot F, and L S is another turning on L, and pointing to the scale M N. R is part of a watch

spring fixed at Y, and pressing gently

Here is a bar

upon the index L s. of iron, at the common temperature of the surrounding air: I lay it in the studs c and D, and adjust the screw P, so that the index Ls may point to O on the scale.

Charles. The bar cannot expand without moving the index F H, the crooked part of which pressing upon LS, that also will be moved if the bar lengthens.

Father. Try the experiment; friction, you know, produces heat; take the bar out of the nuts, rub it briskly, and then replace it.

Emma. The index Ls has moved to that part of the scale which is marked 2 it is now going back. How do you calculate the length of the expansion?

Father. The bar pressed against the index F H at F, and that again presses against Ls at z, and hence they both act as levers.

Charles. And they are levers of the third kind, for in one case the fulcrum is at x, the power at F, and the point z to be moved may be considered as the weight:-in the other, L is the fulcrum, the power is applied at ≈, and the point s is to be moved *.

Father. The distance between the moving point F and H is 20 times greater than that between x and F; the same proportion holds between Ls and LZ: from this you will get the spaces passed through by the different points.

* For an account of the different levers, see Vol. I, Conversations XV and XVI.

Emma. Then as much as the iron bar expands, so much will it move the point r, and of course the point ≈ will move 20 times as much; so that if the bar lengthenth of an inch, the point z would move 28ths, or 2 inches. By the same rule the point s will move through a space 20 times as great as the point z.

Father. There are two levers then, each of which gains power, or moves over spaces, in the proportion of 20 to 1; consequently, when united, as in the present case, into a compound lever, we multiply 20 into 20, which make 400; and therefore, if the bar lengthenth of an inch, the point s must move over 400 times that space, or 40 inches. But suppose it only expandsth part of an inch, how much will s move?