638 Original Articles. [Oct., 
the india-rubber packings were applied; and this indentation was 
greater or less according to the pliability of the insulator. Com- 
municating with the large cylinder, &, £, is a small cylinder, ¢, ¢, fitted 
with a solid plunger. The pressure was applied, through the medium 
of the plunger, by a lever, 1, u (Fig. 3), after the cylinders had been 
filled with water. Up to about 10,000 lbs, pressure per square inch, 
or a pressure equivalent to the weight of a column of water 4°36 miles 
high, the cylinder would stand without leakage; but beyond this 
pressure the water forced its way amongst the packings, and, either 
with or without external leakage, prevented the attainment of any 
higher pressure from the fall of the plunger on its bearings. 
One end of the core was hermetically sealed in all but the earliest 
experiments. The other end was covered with a rounded brass cap, 
and surrounded by a closed box, p, p (Fig. 7), containing dishes, d, of 
concentrated sulphuric acid, an electrometer, e, and a hygrometer, h. 
By means of the acid the atmosphere round the cable was kept in a 
tolerably uniform condition of dryness in a room otherwise damp, and 
the apparatus and surface of the cable maintained under similar con- 
ditions throughout the whole of the experiment. 
The electrometer employed is known as the Peltier’s electrometer. 
Tn this instrument the electricity being simultaneously communicated 
to a fixed bar and a metallic index, the latter is repelled. A directive 
force is given to the index by means of a small magnetic needle, in 
order to retain it at zero when no electric force acts upon it. 
The charge was given from an electrophorus, and was ordinarily of 
such intensity as to deflect the needle through an are of 70°. The fall 
of the needle, from loss of charge, was then watched at intervals as 
nearly uniform as was convenient, until the needle had sunk to 20°. 
Although interesting, it would be unnecessary to give the experi- 
