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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 bathometer, or attraction-meter, was brought out in 1876, and exhibited at the Loan Exhibition in South Kensington. The elastic bottom of the mercury chamber was supported by volute springs which, always having the same tension, caused a portion of the mercury to rise or fall in a spiral tube of glass, according to the variations of the earth's attraction.
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.
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