Charles. You said that Reaumur's thermometer was chiefly used abroad; what is the difference between that and Fahrenheit's?

Father. Reaumur places the freezing point at O, or zero, and each degree of his thermometer is equal to 24, or degrees of Fahrenheit's.

Emma. What does he make the heat of boiling water?

Father. Having fixed his freezing point at 0. and making one of his degrees equal to 24 of Fahrenheit, the heat of boiling water must be 80°.

Charles. Let me see. The number of degrees between the freezing and boiling points of Fahrenheit's thermometer is 180, which, if divided by 24, or 2.25, gives 80 exactly.

Father. You have then a rule by which you may always convert the degrees of Fahrenheit into those of Reaumur: "subtract 32 from the given number, and multiply by the fraction." Tell me, Emma, what degree on Reaumur's scale answers to 167° of Fahrenheit.

Emma. Taking 32 from 167 there remains 135, which multiplied by 4 gives 540, and this divided by 9 gives 60. So that 60° of Reaumur answers to 167° of Fahrenheit.

Charles How shall I reverse the operation, and find a number on Fahrenheit's scale that answers to a given one on Reaumur's?

Father. "Multiply the given number by the improper fraction, and add 32 to the product."

Tell me what number on Fahrenheit's scale answers to 40 on Reaumur's.

Charles. If I multiply 40 by 9, and divide the product by 4, I get 90: to which if 32 be added, the result is 122; this answers to 40 on Reaumur's scale.

CONVERSATION XLV.

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 vII. Fig. 30.) will render the smallest expansion sensible to the naked eyc.

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 ▲ ▲ are fixed three studs B, C, and D, and at в 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 м N. R is part of a watch spring, fixed at y, and pressing gently upon the index L s. Here is a bar 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 L s 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 L s, 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 L s 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

FH at F, and that again presses against L s 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 z, and the point s is to be moved.*

Father. The distance between the moving point F and H is twenty times greater than that between x and F; the same proportion holds between L's and L z, from this you will get the spaces passed through by the different points.

Emma. Then as much as the iron bar expands, so much will it move the point F, and of course the point z will move 20 times as much; so that if the bar lengthen one-tenth of an inch, the point z would move twenty-tenths, or two 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 gain power, or move 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 lengthen one-tenth of an inch, the point s must move over 400 times that space, or 40 inches. But But suppose it only

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

expands th part of an inch, how much will s move?

Charles. One inch.

Father. But every inch may be divided into tenths, and consequently if the bar lengthen only theth part of an inch, the points will move through the tenth part of an inch, which is very perceptible. In the present case the point s has moved two inches, therefore the expansion is equal to ths, orth part of an inch. An iron bar, three feet long, is about th part of an inch longer in summer than in

winter.

2

400

Charles. I see that by increasing the number of levers, you might carry the experiment to a much greater degree of nicety.

Father. Well, let us now proceed to the hygrometer, which is an instrument contrived for measuring the different degrees of moisture in the atmosphere.

Emma. I have a weather-house that I bought at the fair, which tells me this; for if the air is very moist, and thereby denotes wet weather, the man comes out; and in fair weather, when the atmosphere is dry, the woman makes her appearance.

Charles. How is the weather-house construct

ed?

Father. The two images are placed on a kind of lever, which is sustained by catgut; and catgut is very sensible to moisture, twisting and