ometimes also includes the wires for an electric lamp (Fig. 162) used
by the diver at depths to which daylight cannot penetrate.

The pressure on a diver's body increases in the ratio of 4-1/3 lbs. per
square inch for every 10 feet that he descends. The ordinary working
limit is about 150 feet, though "old hands" are able to stand greater
pressures. The record is held by one James Hooper, who, when removing
the cargo of the _Cape Horn_ sunk off the South American coast, made
seven descents of 201 feet, one of which lasted for forty-two minutes.

[Illustration: FIG. 162.--Diver's electric lamp.]

A sketch is given (Fig. 163) of divers working below water with
pneumatic tools, fed from above with high-pressure air. Owing to his
buoyancy a diver has little depressing or pushing power, and he cannot
bore a hole in a post with an auger unless he is able to rest his back
against some firm object, or is roped to the post. Pneumatic chipping
tools merely require holding to their work, their weight offering
sufficient resistance to the very rapid blows which they make.

[Illustration: FIG. 163.--Divers at work below water with pneumatic
tools.]

AIR-PUMPS.

[Illustration: FIG. 164.]

[Illustration: FIG. 165.]

Mention having been made of the air-pump, we append diagrams (Figs. 164,
165) of the simplest form of air-pump, the cycle tyre inflator. The
piston is composed of two circular plates of smaller diameter than the
barrel, holding between them a cup leather. During the upstroke the cup
collapses inwards and allows air to pass by it. On the downstroke (Fig.
165) the edges of the cup expand against the barrel, preventing the
passage of air round the piston. A double-action air-pump requires a
long, well-fitting piston with a cup on each side of it, and the
addition of extra valves to the barrel, as the cups under these
circumstances cannot act as valves.

PNEUMATIC TYRES.

[Illustration: FIG. 166.]

[Illustration: FIG. 167.]

The action of the pneumatic tyre in reducing vibration and increasing
the speed of a vehicle is explained by Figs. 166, 167. When the tyre
encounters an obstacle, such as a large stone, it laps over it (Fig.
166), and while supporting the weight on the wheel, reduces the
deflection of the direction of movement. When an iron-tyred wheel meets
a similar obstacle it has to rise right over it, often jumping a
considerable distance into the air. The resultant motions of the wheel
are indicat