156 Geological Society: — 
riments, we have \ /^H 
and the quantities of electricity produced by sweeping over 
two areas, A and A', are directly as the deflections. 
Let A be the area of the field or portion of field the mean 
intensity (I) of which is being measured, A' and V the same 
quantities for the known field, q, q' the quantities of elec- 
tricity generated in the two cases, 6, 6 r the corresponding de- 
flections supposed both small ; we have 
= AI=1 / 
n»H 
<-«*-W%'> 
and therefore \/a 
l =w v W 
If convenient, 6 and d' may be taken as proportional to the 
numbers of divisions of the scale traversed by the spot of light 
in the two cases. 
I may remark, in conclusion, that the method described in 
(a.) above gives, when the intensity of the field has been accu- 
rately measured by another method, theoretically a method of 
determining a resistance in absolute electromagnetic measure, 
This method is nearly the converse of that given by Weber, 
in which a magnet is made to oscillate within a coil of wire the 
circuit of which is closed. What I venture to propose is to 
hang a coil, the constants of which are known, in a sufficiently 
intense and uniform magnetic field, and find the decrement of 
the oscillatory motion produced by the induction: for it would 
scarcely be possible, I think, to observe with sufficient accu- 
racy the point at which the coil just became " dead beat." 
How far this method may be practicable can hardly be affirmed 
without experiment ; but I think the necessary calculations 
and corrections would be in some degree at least simpler than 
those required for the method given by Weber. 
XXIII. Proceedings of Learned Societies. 
GEOLOGICAL SOCIETY. 
[Continued from p. 71.] 
June 6, 1883.— J. W. Hulke, Esq., F.R.S., President, in the Chair. 
THE following communications were read : — 
1. " The Estuaries of the Severn and its Tributaries, an Inquiry 
into the JNature and Origin of their Tidal Sedimf-nt and Alluvial 
Plats." By Prof. W. J. Bollas, M.A., F.R.S.E., P.G.S. 
Yarious sources have been ascribed to the mud which is so charac- 
