C. K. Wead—Intensity of Sound. 181 
10. Pin C,G.S8. units. P=P’x 981x104. 
11. P computed from equation (7). 
12. The difference between 10 and 11 divided by P iy col. 10. 
_The energy in both prongs of a vibrating fork when its 
amplitude is expressed in divisions of a micrometer; 220 div. 
2 
=lem, The energy =2x4P(=) , where P is to be taken 
from column 10, and z is the amplitude. 
Loss of Energy from the fork.—The usual equation for the 
motion of a body in a resisting medium is 
(12) w= A‘e-# sin {4/p, —t7t—a}, 
where x is the coefficient of damping, and ¢ is the time measured 
from the instant when u=0 (Rayleigh i, 37); A’eé#* will 
correspond to a’ in our equations. The energy at any instant is 
equal to BPia!. =< P(A’ #)2, being sensibly constant through- 
out any one period. Therefore 
(13) Kinetic energy =2T=2P’A'2e“=2P"A2e“'=P "2? =2V, 
where P’” is the number in table I, column 10, divided by 
(220), and A is the amplitude (in micrometer divisions) when 
0; 2V is given in column 18; z is the amplitude in microm- 
‘eter divisions at the time @ 
The rate of loss of energy = 
(14) oe —2xP"A%e"= —2unT=2xV. 
We must then determine x- from the equation z=Ace?™ 
whence x=2 Nap. log. —+1,; it is more convenient in this case 
Zz 
to take the second as the unit of time, rather than the period 
of vibration as is sometimes done in experiments on damping. 
©n which the fork was firmly screwed, was clamped by a car- 
Penter’s clamp. Above, a bar held by another clamp sup- 
: was gummed on 
the tip of the fork and these minute particles furnished fine 
bright points under the microscope, having often a breadth less 
ees zoom. The magnifying power used was about 45, and 
the divisions of the eye-piece micrometer (a glass scale ruled 
zsm™m. The adjustments of the apparatus were easy and direct, 
and the Stability almost perfect. ‘To measure the time a stop 
* Suggested by Terquem, Phil. Mag., March, 1874; C. R. lxxviii, 125, 1874. 
