630 Original Articles. [Oct., 
the Atlantic, but to prove experimentally the properties which peeu- 
liarly belong to the material now in use for the purposes of insulation 
under the varied conditions of pressure, temperature, &c. This being 
the case, and as these experiments were carried to a much greater 
extent as regards pressure, we deem it essential to give them in extenso. 
The following experiments were prosecuted at the request of the 
Commission, with a view to determine how far the different kinds of 
material proposed as insulating coverings for electric submarine cables 
were reliable when placed at the bottom of the ocean under the pressure 
of superincumbent water. It appears that all insulators which have 
been subjected to experiment absorb more or less water under pressure, 
even those that are closest in texture—such as vulcanized india-rubber 
and gutta-percha; and it seems that this absorption increases the 
longer the specimen is retained under water, the greater the pressure 
to which it is subjected, and the higher the temperature of the water 
in which it is immersed. The very limited time which has been 
available for these experiments has prevented my doing more than to 
indicate decisively these general facts, without determining the nume- 
rical relations of the quantities absorbed under different conditions of 
time, pressure, or temperature. But already the experiments point 
out a very important inquiry, some of the methods by which that in- 
quiry may be prosecuted, and some of the conditions which must be 
attended to in order to ensure reliable and corresponding results. 
Generally, in regard to insulating power, the various materials 
tried arrange themselves in the following order of permeability, the 
first absorbing least water, and the last absorbing most :— 
. Chatterton’s compound. 
. Gutta-percha. 
. Masticated india-rubber. 
. Vulcanized india-rubber. 
. Carbonized india-rubber. 
Wray’s compound. 
. Unmasticated bottle india-rubber. 
The experiments on the insulating power of various cores under 
pressure are less complete than those on absorption, and have been 
prosecuted under greater difficulties and with less variety of con- 
ditions. 
So far as the experiments go, however, Wray’s core exhibited very 
high insulating powers, retaining the charge longer than any other 
tried. Next in order to this may be placed a core of pure india-rubber 
coiled in two coats over a wire, but this very rapidly lost its insulating 
power under pressure. Then, a core of pure gutta-percha cured by 
the Mackintosh - process; and the experiments on this are perhaps the 
most satisfactory of the series. The pressure was retained upon the 
cable for 406 hours, in which period it exhibited considerable dimi- 
nution of insulation. A core of twenty alternate coats of gutta-percha 
and Chatterton’s compound also exhibited good insulation unimpaired 
after 170 hours’ immersion. The experiments on a core subjected to 
pressure in an insulating liquid before being placed in our hands gave 
