ll burning.

Francis Bacon, in the _Novum Organum_, lib. ii., makes the following
reference to a machine, or reservoir, of air to which labourers upon
wrecks might resort whenever they required to take breath:--

"A hollow vessel, made of metal, was let down equally to the surface
of the water, and thus carried with it to the bottom of the sea the
whole of the air which it contained. It stood upon three feet--like a
tripod--which were in length something less than the height of a man,
so that the diver, when he was no longer able to contain his breath,
could put his head into the vessel, and having filled his lungs again,
return to his work."

But it was to Dr Edmund Halley, secretary of the Royal Society, that
undoubtedly the honour is due of having invented the first really
practical diving bell. This is described in the _Philosophical
Transactions_, 1717, in a paper on "The Art of Living Under Water by
means of furnishing air at the bottom of the sea in any ordinary depth."
Halley's bell was constructed of wood, and was covered with lead, which
gave it the necessary sinking weight, and was so distributed as to
ensure that it kept a perpendicular position when in the water. It was
in the form of a truncated cone, 3 ft. in diameter at the top, 5 ft. at
the bottom and 8 ft. high. In the roof a lens was introduced for
admitting light, and also a tap to let out the vitiated air. Fresh air
was supplied to the bell by means of two lead-lined barrels, each
having a bung-hole in the top and bottom. To the hole in the top was
fixed a leathern tube, weighted in such a manner that it always fell
below the level of the bottom of the barrel so that no air could escape.
When, however, the tube was turned up by the attendant in the bell, the
pressure of the water rising through the hole in the bottom of the
barrel, forced the air through the tube at the top and into the diving
bell. These barrels were raised and lowered alternately, with such
success that Halley says that he, with four others, remained at the
bottom of the sea, at a depth of 9 to 10 fathoms, for an hour and a half
at a time without inconvenience of any sort.

[Illustration: FIG. 6.--Ordinary Diving Bell.]

This type of bell was used by John Smeaton in repairing the foundations
of Hexham Bridge in 1778, but instead of weighted barrels, he introduced
a force pump for supplying the necessary air. To Smeaton too we are
indebted for the first diving be