MOTION OP A BODY EXPOSED TO BAYS OE HEAT AND LIGHT. 
719 
Calculation of the Intensity of the Force. 
As it will be useful to those who are engaged in the explanation of these phenomena 
to know what is the strength of the force they have to account for, I have made an 
approximate calculation which will at any rate give an idea of the order of magnitude 
of the force. From the largest deflections observed on turning the light on, I calculate 
that the integral couple twisting the mill round a vertical axis is equal to the couple 
produced by the deflection of the dot of light through 125 scale divisions, owing to the 
bifilar suspension. The scale was at a distance of about 1100 scale divisions from the 
mirror. Considering that the dot of light moves through double the angle of deflection, 
I find that 0-06 is too great a value for the sine of the angle of deflection. The couple 
produced by such a deflection is (Clerk Maxwell, ‘ Electricity and Magnetism ’) 
T ab 
a sm a. 
In this equation a and h are the distances of the upper and lower ends of the two 
fibres from each other, h is the vertical distance between the lines joining these upper 
and lower ends, eo is the weight of the suspended body, a the angle of deflection. In 
the experiments we have to give the following values to these constants (the units 
are those of the centimetre-gramme system) : — 
«=0-25, 
5=0-08, 
7i=20, 
*,=32, 
sin a= 0'06 ; 
hence L= 0-00048. 
If is the pressure on unit of area of the wings of the mill, A the area of these 
wings, and l the length of the arm of each wing, 
L=7pA, 
In Mr. Geissler’s light-mill, l is approximately 1-9 and A=6-45 *; hence 
ZA=12-2, 
^=0-000039. 
The pressure on one square centimetre is therefore equal to the weight of the twenty- 
fifth part of a milligramme, or to 0-0006 grain. 
The pressure on the wings of the light-mill was equal to that produced by the weight 
of a film of water equal in thickness to the length of a wave of blue light. This is 
the greatest pressure which I have been able to produce by means of the lime-light. 
* This number is too large. See end of Appendix. 
5 g 2 
