130

NloHOLLS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE

volume of any given mass of air, or other gas, decreases in the same
proportion as the pressure upon it increases.

A glass tube, protected by a brass or copper case, is lowered to the
bottom with the aid of a sinker This tube contains air, and the inside
of it is coated with a chemical preparation called chromate of silver.
The tube is open at the bottom, and water is forced into it according to
the pressure to which it is subjected. The salt water discolours the

Fig. 3.—A Kelvin Sounding Machine, Electric Control.
chemical, turning it into chloride of silver, thus showing how far the
water has entered* the tube. By comparing this with a scale, which
is? supplied with the sounding machine, the depth to which? the tube has
descended is read directly from the scale.
In another make, the water which enters the tube is retained, and
according to the water drawn up in th<* tube so the depth which aie
tube has reached can be read from a scale which is supplied with it.