308 CHARLES DAVISON 
sound-vibrations from both foci were heard simultaneously, and 
the additional strength thus rendered them audible to an 
increased percentage of observers.* 
The variation of other phenomena may be similarly repre- 
sented —such as the frequency of comparison of the sound to 
WIE, BL 
definite types, the audibility of the sound-vibrations before and 
after the shock is felt, the audibility of the loud crashes heard 
when the sound is loudest, etc. The method of course requires 
a very large number of records for its employment ; but, in no 
other way, can the influence of erroneous or defective observa- 
tions be so successfully eliminated. 
CHARLES DavISON. 
Kinc EDWARDS HIGH SCHOOL. 
tPhil. Mag., Jan. 1900, p. 43. 
