the rays converge to the focus of the second
mirror, in which I place one of the bulbs of this thermometer. Now,
observe in what manner it is affected by the caloric which is reflected
on it from the heated bullet. --The air is dilated in the bulb which we
placed in the focus of the mirror, and the liquor rises considerably in
the opposite leg.

[Illustration: Plate III. Vol. I. p. 54
Mr. Pictet's Apparatus for the Reflection of Heat.

Fig. 1.
A.A. & B.B Concave mirrors fixed on stands.
C Heated Bullet placed in the focus of the mirror A.
D Thermometer, with its bulb placed in the focus of the mirror B.
1.2.3.4 Rays of Caloric radiating from the bullet & falling on the
mirror A.
5.6.7.8 The same rays reflected from the mirror A to the mirror B.
9.10.11.12 The same rays reflected by the mirror B to the
Thermometer.]

EMILY.

But would not the same effect take place, if the rays of caloric from
the heated bullet fell directly on the thermometer, without the
assistance of the mirrors?

MRS. B.

The effect would in that case be so trifling, at the distance at which
the bullet and the thermometer are from each other, that it would be
almost imperceptible. The mirrors, you know, greatly increase the
effect, by collecting a large quantity of rays into a focus; place your
hand in the focus of the mirror, and you will find it much hotter there
than when you remove it nearer to the bullet.

EMILY.

That is very true; it appears extremely singular to feel the heat
diminish in approaching the body from which it proceeds.

CAROLINE.

And the mirror which produces so much heat, by converging the rays, is
itself quite cold.

MRS. B.

The same number of rays that are dispersed over the surface of the
mirror are collected by it into the focus; but, if you consider how
large a surface the mirror presents to the rays, and, consequently, how
much they are diffused in comparison to what they are at the focus,
which is little more than a point, I think you can no longer wonder that
the focus should be so much hotter than the mirror.

The principal use of the mirrors in this experiment is, to prove that
the calorific emanation is reflected in the same manner as light.

CAROLINE.

And the result, I think, is very conclusive.

MRS. B.

The experiment may be repeated with a wax taper instead of the bullet,
with a view of separating the light from the caloric. For this purpose a
transparent p