37 6 Rev. Mr . Powell’s experiments relative to 
progress of the direct effect follows a different law from that 
with one screen, and this again from that with two ; and in 
particular, that the maximum in the last case does not occur 
till considerably later than in the former. But if the effects 
were produced by a direct transmission, though their inten- 
sity would be diminished, they would follow a similar law ; 
and the maximum would take place at the same time. These 
results then tend to show that the effect takes place by a 
secondary radiation. But from the inaccurate nature of the 
instrument, I place no reliance on these results alone. 
( 7. ) To examine the point in a more complete and satis- 
factory manner, the two following sets of experiments were 
made with particular care, in which a mercurial thermometer 
was used, and the temperatures of the screens observed by 
means of a small thermometer attached to the face of each 
away from the ball, towards its central part : the bulb being 
kept in contact by the spring of a wire with which the ther- 
mometer was fastened. These thermometers were very 
nearly equal in size ; the diameters of their bulbs being about 
0.175 inch; that of the focal thermometer 0.55 inch, and 
detached about an inch from the mounting. It was gradu- 
ated to quarters of centigrade degrees ; the others only to 
whole degrees. Both the screens were of plate glass : the 
first or nearest the source of heat about T \ inch in thickness ; 
the second about J inch. 
