246 
Proceedings of the Royal Society of Edinburgh. [Sess. 
an excess of pressure perpendicular to the axis. Tidal stress, if effective, 
would give a semi-diurnal effect with minima following the tidal crest ; and 
a fortnightly variation. [Otani ( Proc . Math. Phys. Soc., Tokyo, Jan. 1910) 
has found that compression initially decreases the intensity of magnetisa- 
tion of rocks in the line of compression.] 
22. The preceding investigation must also apply to the piezo-electric 
effect in crystals, and determine relations between the internal electric field 
and the external stress which induces the polarity ; and it will also have 
applications to electro- and magneto-optical effects. 
(. Issued separately July 11 , 1912 .) 
