64 



THE POPULAR SCIENCE MONTHLY, 



Fig. 2. 



Pi 



A 



y 



B 



From the known law regulating the variation in volume of the sub- 

 stance 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 pro- 

 duce 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 in- 

 verted 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 Aime 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. 



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 naercurial thermo- 

 meter. 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 com- 

 pression may be shown as before by an index on the col- 

 umn 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 tempera- 

 ture, as in the polar regions. 



Soundings from vessels in motion may be taken with 

 Massey's machine, in which the friction of the passing wa- 

 ter as it sinks causes a screw-fan to make rotations which 

 are registered by an index. Sir William Thomson has pro- 

 posed 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 com- 

 pression to which an inclosed mass of air has been subjected is mea- 

 sured 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 6r 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 



B 



oio 



