in Chronographic Measurements. 161 



traces are made, but the order in which the styli are used is 

 reversed by a pole-changer in the second trace ; by means of 

 this interchange of styli the latencies are cut out. 



In the diagram, suppose the real beginnings and endings of 



A 



<p. 



<* 





\ 





V* 



r~ 



< — r— * 



1 





sv- 











ll 







«~ r--> 



C 2 B 2 





T>2 



r^ 



■T>" 







1 





— % — _ 







two events to be on the lines A x and A 2 , B 2 and B 2 respect- 

 ively ; let these lengths be called q 1 and q 2 . Suppose d and 

 D 1} C 2 and D 2 to be the markings of the styli in the two 

 traces, and let r and p be the latencies of the two styli ; then 

 in the first experiment the observed period is 



in the second one it is 



qi—p + r, 



q2—r+p; 



by addition and division by 2, 



^ ^ is the mean value of the two readings, the quantities 



r and p being cut out. . 



It is assumed that the latencies of the styli remain constant 

 during the operations. Repeated experiment on this point 

 shows that constancy of the styli may be fully relied on when 

 a suitable battery, such as the gravity DanieU, is used. 



I wish to take this opportunity of correcting an error in the 

 Plate belonging to a paper of mine (p. 377, vol. xxix.). The 

 scribing-point of the tuning-fork should be vertically under 

 or over the points of the styli ; the error was introduced by 

 the faulty arrangement of the instruments when they were 

 photographed. 



Phil. Mag. S. 5. Vol. 30. No. 183. August 1890. M 



