From the known law regulating the variation in volume of the substance with variation in the pressure, the maximum pressure to which the instrument has been exposed can be deduced, and from the known density of the water the height of the column of it which would produce that amount of pressure can be calculated; this height represents the depth to which the instrument has been sunk. Perkins, about 1812, constructed a piezometer, or instrument for measuring pressure, consisting of a glass tube sealed at one end, filled with water, and inverted in a cup of mercury. A steel index placed within the tube rose with the mercury, and was retained by a spring at the highest point reached. Instruments made on this principle were used by him, by Aimé in the Mediterranean, in 1848, and by the United States Coast Survey a few years later. Essentially the same instrument, with certain convenient practical modifications, was used by the author in the Challenger Expedition.
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Another method of measuring the pressure, and through it the depth, of the sea, is by means of an instrument (Fig. 2) much resembling in principle the aneroid barometer. Its simplest form is that usually given to a mercurial thermometer. When the pressure on the outside of the instrument is increased, the bulb tends to collapse, and, flattening, to force the mercury into the stem. The amount of compression may be shown as before by an index on the column of mercury. The use of mercury in this instrument is, however, unsatisfactory, because its contraction under the diminished temperature of the lower depths tends to counteract the effect of pressure in pushing it forward. It is, nevertheless, adapted to waters of a uniform temperature, as in the polar regions.
Soundings from vessels in motion may be taken with Massey's machine, in which the friction of the passing water as it sinks causes a screw-fan to make rotations which are registered by an index. Sir William Thomson has proposed the use of a glass tube, sealed at one end, and coated internally with a chemical preparation, the color of which is changed by the action of sea-water. The sea-water forces itself in as the tube sinks, changing the color of the coating to an extent from which the depth may be calculated. Each of these instruments is good for only one sounding.
The author has patented a device by which the depth of compression to which an inclosed mass of air has been subjected is measured by the water which has gained admittance to the instrument. It is represented in Fig. 3. It consists of a glass tube open at both ends, but capable of being closed by a stopper or other means. At some part of the tube a spout is introduced, and the tube is bent over through two right angles immediately above it. When the
