688 REPORT—1863. 
The gauge-pressure being 60 Ibs. per square inch, one cubic foot of water 
is as destructive as one pound of gunpowder. 
In one of Mr. Biddell’s experiments, the steam-yalve was opened rather 
suddenly, and the steam escaped instantly with a report like that of a very 
heavy piece of ordnance. This is not to be wondered at; forit appears, from 
the comparison above, that the effect was the same as that of firmg a cannon 
whose charge is 44 lbs. of powder. 
Observations on the Electrical Resistance and Electrification of some 
Insulating Materials under Pressures up to 300 Atmospheres. By 
C. W. Siemens, C.E., F.R.S. 
Ir has been repeatedly observed that the insulation-resistance of deep-sea 
cables improves on their being submerged, unless the cables be absolutely 
faulty. But it remained for a long time an open question whether this 
improvement was to be ascribed to the pressure of the water or only to the 
lower temperature at the bottom. This question has, however, been set at 
rest by the electrical tests applied to the Malta-Alexandria cable at the 
Gutta-percha Works, Mr. Reid’s arrangements enabling me to subject the 
coils, during their electrical examination, to a pressure of 600 lbs. on the 
square inch. 
Before pressure was put on, the coils were always immersed for 24 hours 
in water, at a constant temperature of 24°C. The electrical resistance of 
the insulating covering of each knot of the conductor was measured before, 
during, and after pressure, the temperature of the pressure-tank being always 
maintained at 24° C. The ayerage improvement of the insulation under this 
pressure, after the zinc current had been kept to the cable during one minute, 
was found to be nearly 14 per cent., or assuming the improvement to be 
directly proportional to the pressure shown by the gauge, the resistance Rp 
of any coil of this cable under the pressure P, in Ibs., per square inch, whose 
resistance at atmospheric pressure was R, would be expressed by 
Rp=R (14 0:00023P). 
The observations of which the coefficient 0:00023 is a mean would be too 
voluminous to repeat here; the principal part of them is included in the 
weekly reports on the electrical conditions of the cable during its manufacture. 
A pressure-tank has since been constructed at the Gutta-percha Works, in 
which a considerably greater pressure can be attained. I have taken advan- 
tage of this, to have the separate knots of the core of the Oran-Carthagena 
cable, now in course of manufacture, tested under a pressure of 300 atmo- 
spheres—fully equal to that to which the cable will be exposed at the bottom 
of the Mediterranean*. 
The same opportunity has enabled me also to make a few experiments with 
a view to determine the dependence of the insulation-resistances of gutta 
percha, india-rubber, and of a combination of both these materials from ex- 
ternal pressure. I have further extended the experiments to ascertain the 
ratio of increase of the insulation-resistances by polarization or electrification 
under different pressures. 
The results are collected in the following Tables :— 
* The depth of the Mediterranean in that part does not exceed 1500 fathoms, 
