G22 
THE EARL OF ROSSE ON THE RADIATION 
nometer under certain conditions of adjustment, when acted on by the electric currents 
generated by the moon’s heat, which, falling on the large mirror of the 3-foot telescope, 
is concentrated by concave mirrors of short focal length alternately on each of a given 
pair of thermopiles. It was therefore desirable to compare the effect thus produced by 
the moon’s heat with that of a given terrestrial source (say, a blackened tin vessel) acting 
under circumstances as nearly as possible similar to those in the case of the moon. The 
great focal length of the telescope (27 feet) altogether precluded the use of the whole 
instrument for this purpose, so the condensing-mirrors with the thermopiles were 
detached from the telescope, and each separate pile was exposed to the alternate action 
of two circular surfaces of blackened tin backed by water of different temperatures, these 
surfaces being alternately exposed for the interval of a minute through a circular aper- 
ture in a fixed wooden screen. 
In this way, by varying the temperature of the water, a considerable range of readings 
of the galvanometer was obtained ; and these being compared, by means of an empirical 
formula it was easy to calculate the reading for any given temperatures of the tins. 
In this way, by taking into account the moon’s apparent semidiameter, the effective 
area and reflective power of the 3-foot speculum, and the action of the two piles on the 
one hand, and the distance and radius of the circular aperture, the effective area of the 
condensing-mirror, and the one pile on the other, the temperature of the tin vessel 
necessary to produce an effect equal to that of the full moon, when acting under similar 
circumstances, could be at once calculated. 
In the case of the moon-observations we have the following particulars : — 
in. 
Full area of speculum =973T2 
Deduct for direct obstruction . . . = 116-80 
856-32 
The assumed reflective power of the speculum; =0-5325 
Proportion of the cone of rays not intercepted by pile and holder = 0-900G4 
Hence the 3-foot speculum may be replaced by one of perfect 
reflective power whose area =410-7 sq. in. 
Moon’s mean apparent semidiameter =15'-557 
If we take F as the unit, 
the moon’s area=T X 15*557 2 , 
and the source of lunar radiation for a unit of excess of temperature 
=t.(15-557) 2 x410-7=M. 
Again, the distance of the hot tin from the condensing-mirrors was 111 inches, its radius 
6 inches, and the unobstructed area of the condensing-mirror 8‘597 inches, therefore 
source of heat for comparison='v(0 , 054054x3437‘75) 2 x8-597=T; hence 
T 
