ve
the ocean to the east or the north, the winds blowing thence would be
the rainy winds, while south-westerly winds might bring hot and dry
weather.

THE DIVING-BELL.

Water is nearly 773 times as heavy as air. If we submerge a barometer a
very little way below the surface of a water tank, we shall at once
observe a rise of the mercury column. At a depth of 34 feet the pressure
on any submerged object is 15 lbs. to the square inch, in addition to
the atmospheric pressure of 15 lbs. per square inch--that is, there
would be a 30-lb. _absolute_ pressure. As a rule, when speaking of
hydraulic pressures, we start with the normal atmospheric pressure as
zero, and we will here observe the practice.

[Illustration: FIG. 158.--A diving bell.]

The diving-bell is used to enable people to work under water without
having recourse to the diving-dress. A sketch of an ordinary
diving-bell is given in Fig. 158. It may be described as a square iron
box without a bottom. At the top are links by which it is attached to a
lowering chain, and windows, protected by grids; also a nozzle for the
air-tube.

[Illustration: FIG. 159.]

A simple model bell (Fig. 159) is easily made out of a glass tumbler
which has had a tap fitted in a hole drilled through the bottom. We turn
off the tap and plunge the glass into a vessel of water. The water rises
a certain way up the interior, until the air within has been compressed
to a pressure equal to that of the water at the level of the surface
inside. The further the tumbler is lowered, the higher does the water
rise inside it.

Evidently men could not work in a diving-bell which is invaded thus by
water. It is imperative to keep the water at bay. This we can do by
attaching a tube to the tap (Fig. 160) and blowing into the tumbler till
the air-pressure exceeds that of the water, which is shown by bubbles
rising to the surface. The diving-bell therefore has attached to it a
hose through which air is forced by pumps from the atmosphere above, at
a pressure sufficient to keep the water out of the bell. This pumping of
air also maintains a fresh supply of oxygen for the workers.

[Illustration: FIG. 160.]

Inside the bell is tackle for grappling any object that has to be moved,
such as a heavy stone block. The diving-bell is used mostly for laying
submarine masonry. "The bell, slung either from a crane on the masonry
already built above sea-level, or from a specially fitted barge, comes
into