138 Notices respecting New Books. 
to penetrate them. Suppose then a plate of polished metal, to 
attract caloric from a distant object. The particles of calorie 
will strike upon its surface, but the greater part will be unable 
to enter it. That part of the ray of calorie therefore, whieh 
cannot penetrate, impinging upon the polished metal with a 
force greater than the attraction at the surface, and its elasticity 
being perfect, it will rebound or be reflected, with a force equal 
to the excess ; the attraction acting pon it on its return, with 
acontinually decreasing, retarding power. If, however, another 
body be placed so as to exercise its attraction in the course of 
the reflected ray, the retarding attraction will be opposed by the 
force of that attraction, and the course of the returned ray will 
be continued. If the heated body be placed in the focus of a 
concave metallic mirror, the caloric would be attracted by it, 
and almost the whole quantity attracted, would be reflected from. 
its polished surface in parallel lines. The metal, however, does 
not act with an equally retarding attraction upon the ray of ca- 
Toric, because a concave mirror will always attract with the 
greater power, objects in the line of its focus. This will be very 
evident, if that ray be considered, which impinges upon the ex- 
treme circumference of the mirror, when it will be perceived, 
that on its return, at the same distance from the part on which 
it struck, it will be much less under the influence of the attrac- 
tion of the mirror, than in the heated body situated in the foeus. 
This difference of attraction must have a very considerable 
effect ; for 2 sinall force will convey a ray of caloric to a great 
distance, as may be easily imagined from its usual wonderfully 
rapid motion, And if a small direct force be sufficient, a small 
excess over a counteracting force is equally effectual. If the 
retarding attraction therefore, of the first mirror, with all its op- 
posing circumstances, be but in a smal degree less than its ac- 
celerating attraction, caloric may be conveyed to a considerable 
distance. But the small force with which the caloric passes 
from the mirror, beyond the distance of the heated body, being 
ouly the excess of the direct over the retarding attraction, does 
not lessen the quantity, in the same degree, as it would if it: 
arose from a small power of attraction. The quantity passing 
is that which is attracted by the mirror from the heated body, 
with the deduction of that quantity which the mirror itself re- 
tains, and that which will be retained by the attraction of the 
atmosphere, and other such circumstances. Ifa second mirror 
be then placed opposite to the first at a moderate distance, the 
rays will impinge upon it, and most of them be reflected to its 
focus. The second mirror will not only reflect the rays which 
strike upon it, it will also assist their progress. by its own attrac- 
tion 
