160 Mr. F. J. Smith on Latency of Electromagnetic Styli, 

 Now the right-hand sides of (5) may be written 



d /dP dR\ d /dQ dF\ 



/dP_dR\d L /alQ 

 \ dz dx J dy \dx 



dz \ dz dx J dy \dx dy J 



dx\dx dy J dz\dy dz ) 



d /dR _dQ\_ ±(d?__ dR\ 

 dy\dy dz J dx\dz dx J' 



Comparing these equations with (23), it appears that 

 Boussinesq's terms conld not be deduced by means of any 

 assumption respecting the relation between electromotive force- 

 and electric displacement. 



In order to apply Boussinesq's theory of quartz to the 

 theory of double refraction of Lord Rayleigh and Sir W. 

 Thomson, we must put p 1 =p 2 = in (23) ; and the last terms 

 on the right-hand side of (20) will become 



d d v d 3 u 



P 7z~dt 2 > ~ P dz~dt^ 



Solving the resulting equations in the same manner as 

 before, we shall obtain 



(V 2 -a 2 )(V 2 -aV-c 2 P) = A ^^f n " (V 2 -c 2 P), . (24) 



the right-hand side of which is different from either of our two 

 former results. 



XVII. On a Method of Eliminating the Effects of Latency of 

 Electromagnetic Styli, in Chronographic Measurements. By 

 Frederick J. Smith, Trinity College, Oxford*. 



WHILE working with the chronograph, a description of 

 which appeared in the Philosophical Magazine for 

 May, I have introduced a method of taking time-traces, by 

 which instrumental error is greatly reduced, when styli are 

 used which differ in their " latencies." It is as follows : — Two 



* Communicated by the Author. 



