ON TEE GRANITES OF DONEGAL. 163 
is immediately under the free end of the springs, and is armed with a small 
platinum knob or contact-piece. The three springs are also all armed with 
platinum contact-pieces, all in a line one above the other (fig. 6). When 
the finger-piece, T, is pressed down, 1 and 2 are first joined, and then 3 
and 4; 3 is insulated from 2 by the vulcanite, Q. All the connexions per- 
manently made, under the board, are shown in fig. 2. Those which have no 
sensible resistance are stout copper bars, and form the bottoms of the mer- 
cury-cups: those of which the resistance is immaterial are made of wire, 
insulated by gutta percha, and are simply shown as dotted irregular lines in 
fig. 2; they will be found, on comparison, to correspond with the thin lines 
on fig. 7. It will also be found that all those parts shown by thick lines in 
the diagram are made by thick bars or rods and merecury-cups.  _ 
Three sets of arms, C A, C, A,, C,A,, are provided; the shortest pair is 
first used, and U adjusted by the slide, H, till the galvanometer does not de- 
flect when T is pressed down. The commutator, D, is then turned round, 
and U adjusted afresh. The coil, R, is then altered according to the two 
positions of U, and this process repeated, using the second and third pair of 
arms as required, until the desired approximation between Rand § has been 
obtained. An astatic galvanometer, with a very long coil, will, for most 
purposes, give the best results; and one or two elements will be found a 
sufficient battery. The construction of R and S$ recommended, and the pre- 
cautions to ensure perfect equality of temperature, will form part of next 
year’s Report. 
The apparatus, although specially designed for the production of equal 
coils, is applicable to ordinary measurements of resistances by comparison 
with a set of resistance-coils; for this purpose the terminals of the resist- 
ance-coils should be put in the place of the standard 8, and any conductor of 
which the resistance is to be measured in the place of R. If a comparison 
by equality is to be made, the wire WX can be used as already described ; it 
is, however, frequently desirable to make a comparison with one arm ten- 
fold or a hundredfold greater than the other, by which means measurements 
of resistances can be made ten or a hundred times greater or smaller than 
could be done if equality alone between R and 8 were measured ; for this pur- 
pose the three pairs, A C, A, C,, A, C,, are made exactly decimal multiples one 
of the other, and then, by taking A and C,, or A and C,, &c., in the cups aa, 
and ¢¢,, the required decimal ratio is obtained. The resistance of the wire 
WX would, however, falsify this ratio, and it is eliminated by a simple 
copper rod, which is placed for the purpose between the two cups ¢e,, and 
maintains the whole wire WX at sensibly one potential. The commutator 
also is useless in measurements of this kind, and should be left untouched in 
the position shown in fig. 1. 
The apparatus exhibited was manufactured for the Committee by Messrs. 
Elliott Brothers, of London, and gives excellent results, 
Preliminary Report of the Committee for Investigating the Chemical 
and Mineralogical Composition of the Granites of Donegal, and the 
Minerals associated with them. 
In accordance with the resolution of the General Committee at the Man- 
chester Meeting, the Committee, consisting of Sir R. Griffith, the Rev. Prof. 
Haughton, and Mr. Scott, proceeded to investigate “ the chemical and mine- 
M 2 
