724 ON THE MOTION OE A BODY EXPOSED TO EATS OE HEAT AND LIGHT. 
the mill, we must divide the couple L by the length of the arms and the area of the 
wings. The length of the area was approximately 1*9 and the area 4-84 ; hence the 
pressure on a square centimetre during the experiments was 
0000030 grammes, 
which is equal to the weight of a layer of water covering the surface, and equal in thick- 
ness to the length of a wave of ultra-violet light. The pressure is about double that 
determined by Mr. Crookes in one of his experiments*. Considering the greater 
intensity of the light used in my experiments, the results agree pretty well with each 
other. 
In the original paper the area of the wings was erroneously given as 6*45 ; the pressure 
was therefore found too small in the proportion of 4*84 to 6 ’45. The pressure deduced 
by the rough method of the original paper should therefore have been 
^=0-000052. 
The number only professed to be an outer limit to the pressure, the real pressure being 
necessarily smaller. The intensity of the light used in the more careful experiments 
described in these pages was decidedly smaller than that used in the original experi- 
ments. Under these circumstances I think the agreement between the two numbers is 
as close as could be expected. 
* Lecture delivered at the Eoyal Institution, February 11 ; reprinted, ‘ Quarterly Journal of Science,’ 
April 1876. 
