879.] 
Temperature of the Sun. 
657 
Heat Comparisons. 
After many visits to the works, much trouble and 
repeated failures due to the difficulties of working in such 
novel circumstances, I secured a series of trustworthy 
measures, in May last, both of heat and light. I describe 
my apparatus here in principle, not in detail ; and I omit 
many preliminary experiments, as well as some minute 
corrections applied for small instrumental errors, giving my 
results in general terms. One difficulty attending a simul- 
taneous comparison was to obtain a station looking into the 
“ converter ” at the time it was inclined and pouring, and 
yet necessarily outside the building in the sunlight. To do 
this, I stood in a window (whence the sash had been 
removed) of the west wall, sixty-one feet from the “ con- 
verter ” mouth. A platform was erected here for my 
apparatus, part of which was clamped to the wall itself ; 
but though this was the best point of observation, the noise, 
the shower of sparks driven over the instruments from 
within by the blast at each “ pour,” and the rain of wet 
soot without which fell thick at times on apparatus and 
observer from the combined steam and smoke of adjacent 
chimneys, made the task of observation another thing from 
what it is in the quiet of a physical cabinet. 
From this window-station, a porte-lumiere reflected the 
sun’s rays, so that traced through the dusty air the beam 
was seen to enter the “ converter ” mouth, or fall on the 
stream which ran from it. In the path of this beam was 
a cylinder, containing within a double enclosure an Elliott 
thermopile of forty small elements, similar to that I had 
used for some years on the sun, and surrounded by all the 
precautions against air-currents and extraneous influences 
taught me by experience. The pile exposed both faces at 
once, one to the furnace, the other to the reflected sunbeam ; 
and a Thompson reflecting galvanometer read by an 
assistant, and placed at a considerable distance from any 
moving iron, gave prompt evidence as to which face was 
hotter. 
The angular area, subtended at the pile by the fluid 
metal, was always many times that subtended by the sun’s 
disk, and there was no lens or medium of any kind (except 
air) between the “ converter ” mouth and the pile. Sup- 
posing, then, the metal to have only presented a disk equal 
in angular diameter to that of the sun, if the needle remained 
stationary, it is plain that each was sending an equal amount 
ot heat, and that any square foot of the solar surface was 
