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Art. II. — Observations on Radiant Heat. By William 
Bitchie, a. M. Rector of the Academy at Tain. 
The theory of radiant heat, which we have illustrated in the 
last number of this Journal % will enable us to explain, in a novel 
manner, some of the most striking facts discovered by Professor 
Leslie, and accounted for by that ingenious philosopher upon 
the supposition, that caloric is conveyed from one body to an- 
other by the pulses excited in the ambient air. 
If a polished metallic surface be rubbed with sand-paper, or 
ploughed into fine parallel fiutings, its radiating power will be 
greatly increased. 
Let the surface of the body be fluted by the point of an equi- 
lateral triangle, as is represented in the figure. Draw CD per- 
pendicular to AB. Then the quantity A. D R 
of caloric radiated from AC, in lines pa- J 
rallel to CD, is to the quantity radiated 
from AC, in lines perpendicular to AC, as the sine of ^ ACD, 
is to radius* But the angle ACD is 30®, the sine of which is 
equal to half the radius. Hence the quantity of heat thrown off* 
from AC, in lines parallel to CD, is equal to the quantity thrown 
off* from AD, in the same direction. Now, as the same may be 
demonstrated with regard to CB, it follows, that the quantity of 
caloric which radiates from both sides of the prismatic furrows, 
is equal to the quantity radiated from the space AB of the plane 
polished surface, the temperature of the body being die same. 
But the caloric radiated from the lower half of AC, at right 
angles to that surface, will impinge upon CB, and be reflected 
in lines parallel to CD. Now, as the same thing takes place with 
the heat thrown off* from the lower half of CB, it follows, that the 
caloric radiated from AC and CB, and reflected in lines parallel 
to CD, is equal to the quantity thrown off* in the same direction 
from the original plane surface AB, provided none of it had 
been absorbed by the sides of the flutings. But since a metallic 
surface is an excellent reflector, it follows, that the portion ab- 
sorbed will be very small when compared with the whole quan- 
tity radiated ; and, consequently, the effect produced upon the 
bulb of the differential thermometer will be nearly doubled. 
* No. XXII. (Oct, 1824), p. 281. 
