with Messrs Siebe, Gorman & Co., with a view to solving this
problem, and their efforts have been attended with some considerable
success. Dr J. S. Haldane has also carried out practical experiments for
the British Admiralty, and under his supervision two naval officers have
succeeded in reaching the unprecedented depth of 210 ft., at which depth
the pressure is about 90 lb. to the square inch.

_Diving Bells._--Every one is familiar with the experiment of placing an
inverted tumbler in a bowl of water, and seeing the water excluded from
the tumbler by the air inside it. Perhaps it was to some such experiment
as this that the conception of the diving bell was due. As is well
known, the pressure of water increases with the depth, and for all
practical purposes this pressure can be taken at 4-1/4 lb. to every 10 ft.
The following table shows the pressure at different depths below the
surface of the water:--

Depth. Pressure.

20 ft. 8-1/2 lb to the sq. in.
40 " 17-1/4 " "
80 " 34-3/4 " "
120 " 52-1/2 " "
160 " 69-3/4 " "
200 " 87 " "

If a diving bell be sunk to a depth of, say, 33 ft., the air inside it
will be compressed to about half its original volume, and the bell
itself will be about half filled with water. But if a supply of air be
maintained at a pressure equal to the depth of water at which the bell
is submerged, not only will the water be kept down to the cutting edge,
but the bell will be ventilated and it will be possible for its
occupants to work for hours at a stretch.

Tradition gives Roger Bacon, in 1250, the credit for being the
originator of the diving bell, but actual records are lost in antiquity.
Of the records preserved to us, probably one of the most trustworthy is
an account given in Kaspar Schott's work, _Technica curiosa_, published
in the year 1664, which quoted from one John Taisnier, who was in the
service of Charles V. This account describes an experiment which took
place at Toledo, Spain, in the year 1538, before the emperor and some
thousands of spectators, when two Greeks descended into the water in a
large "kettle," suspended by ropes, with its mouth downwards. The
"kettle" was equipoised by lead fixed round its mouth. The men came up
dry, and a lighted candle, which they had taken down with them, was
sti