136 
tangents; and in absolute measurements it has the dis- 
advantage that the diameter of the circle of ribbon cannot 
be taken as the virtual diameter. It appeared to me that 
the best plan for a galvanometer would be to use a wire of 
about one-tenth of an inch diameter, surrounding a short 
needle, and to supply the small correction needed to the 
tangents of the deflections of the latter. 
Calling, then, 6 the angle of deflection, l the rnagnetical 
length of the needle (generally about four-fifths of the actual 
length), and d the diameter of the coil, we have 
{ (2sin0) 2 — (cos 6) 2 \ tan0, 
or, more elegantly and simply, as suggested to me by Pro- 
fessor Jack, 
I(4tan 2 0— l)f sin20, 
A ct 
for the correction to be supplied to the tangent of the angle 
observed. This correction is additive at great deflections, 
and subtractive at small ones, whilst at 26° 34' the correction 
vanishes. 
It seems, therefore, desirable that in exact researches a 
needle of small length should be employed, and the apparatus 
so arranged as to give a deflection somewhere about the 
above-indicated angle, where the correction, if needed at all, 
would be of trifling amount. 
“ On a Section of the Drift Deposits in the Banks of the 
Kibble, near Balderston Hall,” by Edward Hull, B.A., 
F.G.S. 
Since my paper on the “ Drift Deposits in the neighbour- 
hood of Manchester,” &c., was published in the memoirs of 
the Literary and Philosophical Society of Manchester,* I have 
had opportunities of considerably extending my acquaint- 
* Mem. Lit. & Phil. Soc., Manchester, vol. ii., 3 series, p. 449. 
