430 Journal of the Asiatic Society of Bengal. [N a XIV, 
2 ke ; 
Now n=, and re (5) , where 8=logarithmie decre- 
ment of the amplitude. 
_ &_[2e : B we 2 ‘ 
I can be easily calculated from the weight and the 
dimensions of the system. jp can therefore be easily calculated 
from formula (iv). 
In our experiment /=2°65 c.m. and a=6°27 x 107° so that 
a deflection of (1 mm.) at‘a distance of 1 metre corresponded to 
a total pressure of 
6:27 x 1075 
2°65 
The time period was 32 seconds and the logarithmic decrement 
was £='310, and J =1:67 units. 
= 2°36 x 107° dynes. 
MEASUREMENT OF ENERGY. 
_ Nichols and Hull’s method was more ingenious. 
disc of silver of the same size as the vane was coat 
lampblack. Two holes were bored on the sides through 
& copper-constantan couple passed. The other end 
couple passed through a sensitive galvanometer. This i st 
atus was previously standardised by putting it in differen 
baths. The light was allowed to fall on the disc for 80 
A thio 
ed with 
which 
the 
+ ’ 7 as 
The source of light in Lebedew and sae ome sa 
y- : . 
experiments we used the arcs but in the latest experiment 1 
source of light was a (3000 c.p.) Tungsten filament lamp * 
plied by Messrs, Westinghouse & Co. The light fror tal 
in 2 horizon’ 
a distance 
focus lens of 6-5 cms. apperture. By adjusting the | 
filament was completely focussed on the vanes. An 
limit to the amount of energy falling on the vane 
can be thus obtained. By means of an ammeter we 
