Page:Heroes of the telegraph (IA cu31924031222494).djvu/134

 in 1863. The effect of pressure up to 300 atmospheres was observed, and the fact elicited that the inductive capacity of gutta-percha is not affected by increased pressure, whereas that of india-rubber is diminished. The electrical tests employed during the construction of the Malta and Alexandria cable, and the insulation and protection of submarine cables, also formed the subject of a paper which was read before the Institution of Civil Engineers in 1862.

It is always interesting to trace the necessity which directly or indirectly was the parent of a particular invention; and in the great importance of an accurate record of the sea-depth in which a cable is being laid, together with the tedious and troublesome character of ordinary sounding by the lead-line, especially when a ship is actually paying out cable, we may find the requirements which led to the invention of the 'bathometer,' an instrument designed to indicate the depth of water over which a vessel is passing without submerging a line. The instrument was based on the ingenious idea that the attractive power of the earth on a body in the ship must depend on the depth of water interposed between it and the sea bottom; being less as the layer of water was thicker, owing to the lighter character of water as compared with the denser land. Siemens endeavoured to render this difference visible by means of mercury contained in a chamber having a bottom extremely sensitive to the pressure of the mercury upon it, and resembling in some respects the vacuous chamber of an aneroid barometer. Just as the latter instrument indicates the pressure of the atmosphere above it, so the bathometer was intended to show the pull of the earth below it; and experiment proved, we believe, that for every 1,000 fathoms of sea-water below the ship, the total gravity of the mercury was reduced by $1⁄3200$ part. The bathometer,