723 
MOTION OF A BODY EXPOSED TO EATS OE HEAT AND LIGHT. 
Putting the value, and remembering that K+^=2IX, we find 
L =B ((?->•)+»■ 
nrfi 1 
=BIT 
(16) 
(A 2 + ra 2 ) sin 2 « J 
We now turn to the determination of B from the successive elongations of the vessel, 
and from the approximate time which was observed we find A=99’5 in divisions of the 
scale. The mean of two experiments gave for the first elongation 124-3. Hence we 
calculate from (14) and (15) 
t,= 9T5 
and 
— a=68° 33'. 
From this we get B = 92’6 in divisions of the scale, 
we find 
B=0-02953. 
The distance of the scale from the mirror was 1526 divisions of the scale. In the calcu- 
lation of B we had to apply a correction, because the scale did not stand parallel to the 
mirror when the mirror was at rest. This was done by multiplying B with 0-9771. 
(This number was obtained by measuring the angle of inclination of the scale.) 
Putting this value of B into (16), we get 
L=0-0002714. 
From (10) we get ^=0T1795 ; and we have therefore now determined all the con- 
stants of the equation. In order to see in how far the values obtained correctly repre- 
sent the experiments, the second elongation was calculated by the formula to be 172-3 
divisions of the scale. By experiments it was found to be 171. As the second elonga- 
tion did not enter into any determination of the constants, the agreement seems 
satisfactory. 
Calculating the position of rest from the successive elongations when the mill was 
turning, and afterwards when the mill was at rest, a difference of 1-3 division of the 
scale was noticed. The vessel appeared to be deflected 1‘3 division of the scale in a 
direction opposite to that in which the mill was turning. Considering the difficulty 
which besets the exact determination of the zero-point when the light is turned on the 
mill, owing to changes in the intensity of the light, external air-currents, &c., we may 
regard the two positions of rest to be, as far as our experiment could determine them, 
identical. The permanent deflection either way, if it existed, must, we may say with 
certainty, have been smaller than 5 divisions of the scale. 
The moment of any external force, if existing, must therefore have been smaller than 
0-000013, or less than 5 per cent, of the internal force. 
In order to get a numerical value for the pressure on the unit of area of the wings of 
