Geology. 319 
in order to produce the same current by means of induction, has 
been led to a determination of the mechanical equivalent of heat. 
He made use of the magneto-electric machine of constant current 
of Siemens and Halske, which is described in the twelfth volume 
numbers) is the value of the induction work per second in order 
to produce the electromotive force of one Daniell cell in a circuit 
of 1 S.E resistance. To produce the electromotive force of one 
Bunsen cell, 0-4 mkg is necessary. Reckoning a horse power at 
2 
75 mkg, the expression 0°0053 ~ represents the induction work 
. 81. a. 
8. Mechanical Equivalent of Heat.—In the proceedings of the 
American Academy of Arts and Sciences, Part I, 1879, is an 
the temperature rises. Complete details are given of the appara- 
tus emplo : Dg 
4, ispersion Photometer.—Messrs. Perry and A¥YRTON 
describe a new photometer for measuring very strong light. The 
light is made to pass through a concave lens and is thus dispersed 
at a short range over a large surface. The authors believe that 
the amount of light absorbed by the lens can be accurately esti- 
mated, and think that measures can be made more accurately in 
this way than in the methods of measuring lights from a great 
distance.— Phil. Mag., Feb., 1880, p. 117. J. T. 
Il. GroLoey. 
