e in the air thus confined although thrust to great depths under
water. But it could not live long, because air becomes unfit for use
after being breathed a certain time, and cannot sustain life. Hence, if
we are to preserve the life of our fly, we must send fresh air down to
it.

The first diving-bells were made so large that the air contained in them
sufficed for a considerable period--an hour or more. When this air had
lost its life-sustaining qualities, the bell had to be drawn up and the
air renewed. This was so inconvenient that ingenious men soon hit on
various plans to renew the air without raising the bells. One plan,
that of Dr Halley, was to send air down in tight casks, which were
emptied into the bell and then sent up, full of water, for a fresh
supply of air, while the foul air was let out of the bell by a valve in
the top. Another plan was to have tubes from the bell to the surface by
which air was made to circulate downwards, at first being forced down by
a pair of bellows, and afterwards by means of air-pumps.

Round the inside of the bell ran a seat for the divers. One or more
holes fitted with thick plate-glass, gave them light and enabled them to
use the various tools and implements required in their vocation. From
some of these bells, a man could be sent out, when at or near the
bottom, having on a water-tight head-piece connected by a tube with the
air inside the bell. He could thus move about with more freedom than
his comrades inside, but of course could not travel further than the
length of his tube, while, being wet, he could not endure the cold for
any great length of time.

As time went on the form of the bell was improved until that of a square
or oblong box of iron came to be generally adopted. The bell now in use
is that which was made in 1788 by the celebrated engineer Smeaton, who
applied the air forcing-pump to it, and otherwise brought the machine to
a high degree of perfection. He used it with great advantage in the
works at Ramsgate harbour, and Smeaton's diving-bell, improved by
Rennie, has continued in constant and general use on all submarine works
until a very recent period. It has now been almost entirely
superseded--except in the case of some special kinds of work--by the
diving-dress--the value and the use of which it is the province of our
tale to illustrate and expound.

In regard to the diving-dress, we may say that it has grown out of the
"aquatic armour" o