The Diving-bell is an apparatus extensively employed by the engineer in
many difficult operations connected with his arduous pursuits. Not only
are rocks blasted by the force of gunpowder _above_ the water, but
below; and the spirit-level of the plumb-line, and the nicely fitted
joints of the stone-mason, are quite as much attended to at any
reasonable depth below the surface of the _sea_, as in the mightiest
works on the _land_. Low water-mark is no longer a limit to the
operations of the engineer; and he now lays his foundations on the edge
of a sub-marine precipice, with the same security and precision as
characterise his noblest operations on the ground.
The diving-bell commonly in use resembles nearly a large box deprived of
its bottom. Its ordinary length is about six feet, its breadth five and
a half, and its height four feet and a half. To avoid the necessity of
fastening weights to make it descend, it is formed of cast-iron; and
being made in one piece, and very thick, there is no danger of the water
forcing itself through the sides or top. It is also air-tight. The
thickness of the sides of the bell prevents also its being fractured,
should it by any accident receive a heavy blow.
In the top of the diving-bell is a round aperture, communicating by a
number of small circular holes with the interior, where the holes are
all covered and closed by a piece of thick leather, which acts as a
valve, and admits air. A strong leather hose is screwed on to the
external aperture, and from two holes near its sides rise two strong
chains, uniting in a ring, by which the whole machine is to be
suspended. In the top also are cemented twelve very thick lenses, for
the purpose of admitting light. At the ends of the bell are two seats,
placed at such a height, that the top of the head is but a few inches
below the upper part of the bell; and in the middle, about six inches
above the lower edge, is placed a narrow board, on which the feet of the
divers rest. On one side, nearly on a level with the shoulders, is a
small shelf, with a ledge, to contain a few tools, chalk for writing
messages, and a ring, to which a small rope is tied. A board is
connected with this rope; and after writing any orders on the board with
a piece of chalk, on giving it a pull, the superintendent above, round
whose arm the other end is fastened, will draw it up to the surface,
and, if necessary, return an answer by the same conveyance. “Our
compliments to our friends above water,” was the little memorandum
written by the author of this brief notice, when he formed one of a
happy party at the bottom of the sea. “Health and prosperity to the
ladies and gentlemen inhabiting the region of fishes,” was the answer
which was received to it in less than three minutes.
On the top of the bell, on the inner side, it is usual to have some
contrivance, by which stone or other bodies may, if necessary, be
suspended from the bell. The weight of the whole apparatus is about four
tons. The leather hose is connected with a double condensing pump,
usually worked by four men. In order to give motion to the bell, it is
suspended by a windlass purchase-tackle, which is fixed on a moveable
platform, having four wheels; these wheels move along an iron railway,
which is itself fixed on another platform, having by the same means a
motion in a direction transverse to the former, at right angles to each
other. Thus by two iron railways, established on beams and supported by
piles the lower being fixed in the direction of the length of the wall,
and the upper being on the lower moveable plane, it is possible to give
the bell any position that may be required.
There are many prejudices against diving-bells, just as there were once
against stage-coaches, steam-boats, and travelling by the power of
vapour on rail-roads Some of our readers, who have possibly misgivings
respecting the safety of a descent in a diving-bell may be induced to
enter one, after they have read the following account by Mr. Babbage of
his descent with Mr. Harvey in a diving-bell at Plymouth:--“To enter the
bell, it is raised about three or four feet above the surface of the
water; and the boat, in which the persons who propose descending are
seated, is brought immediately under it. The bell is then lowered, so as
to enable them to step upon the footboard within it; and having taken
their seats, the boat is removed, and the bell gradually descends to the
water.
“On touching the surface, and thus cutting off the communication with
the external air, a peculiar sensation is perceived in the ears; it is
not, however, painful. The attention is soon directed to another object.
The air rushing in through the valves at the top of the bell overflows,
and escapes with a considerable bubbling noise, under the sides. The
motion of the bell proceeds slowly, and almost imperceptibly; and on
looking at the glass lenses close to the head, when the top of the
machine just reaches the surface of the water, it may be perceived, by
means of the little impurities which float about in it, flowing into the
recesses containing the glasses. A pain now begins to be felt in the
ears, arising from the increased external pressure; this may sometimes
be removed by the act of yawning, or by closing the nostrils and mouth,
and attempting to force air through the ears. As soon as the equilibrium
is established the pain ceases, but recommences almost immediately by
the continuance of the descent. On returning, the same sensation of pain
is felt in the ears; but it now arises from the dense air which had
filled them endeavouring, as the pressure is removed, to force its way
out.
“If the water is clear, and not much disturbed, the light in the bell is
very considerable; and, even at the depth of twenty feet, was more than
is usual in many sitting-rooms. Within the distance of eight or ten
feet, the stones at the bottom began to be visible. The pain in the ears
still continued at intervals, until the descent of the bell terminated
by its resting on the ground.
“Signals are communicated by the workmen in the bell to those above, by
striking against the side of the bell with a hammer. Those most
frequently wanted are indicated by the fewest number of blows; thus a
single stroke is to require more air. The sound is heard very distinctly
by those above.”
Considering the extensive employment of the diving-bell, few serious
accidents have occurred. Some years since, the bell at Sheerness rested
on the top of an old pile. The men within repeatedly gave the signal to
lower instead of to raise the bell. This being obeyed, it fell over, and
two out of three who were in it, were drowned; the third came to the
surface, and was saved. On another occasion at Blackwall, a bell, of
which trial was being made on ship-board, was mismanaged. There were
three persons below when the water began to fill it. One of them, with
great presence of mind, dived underneath the edge, came to the surface,
gave the alarm, and saved his companions.
Several diving-bells have been constantly employed during the last five
or six years at Plymouth, at every variety of depth, and no accident
whatever has happened with them. The truth is, the whole apparatus is
now so admirably constructed, and the mode of applying it so perfectly
understood, that nothing but the grossest ignorance and mismanagement
could produce inconvenience. The workmen constantly enter it without
fear, and remain several hours at the bottom of the sea, adjusting
stones of enormous weight, with the same accuracy and precision as they
would do above. There is even a rivalry among the men who shall descend
on account of the very small extra pay allowed for working in it. In
South America, a large proportion of the treasure sunk in the Thetis has
been recovered by means of the diving-bell.
The following is a description, abridged from Brewster’s Cyclopædia, of
Spalding’s Diving Bell, which is represented in the following cut:--
A, B, C, D, represents the body of the bell, which is suspended by four
ropes, _a_, _a_, uniting together at their junction with the great rope
E. _b_, _b_, ballast weights; these keep the mouth of the bell, C, D,
always parallel to the surface of the water. By these weights alone,
however, the bell would not sink; another is therefore added, F, which
by means of a pully can be raised or lowered at pleasure. In descending,
this balance weight hangs considerably below the bell. In case the edge
of the bell is caught by any obstacle, the balance weight is immediately
lowered down, so that it may rest upon the bottom. By this means the
bell is lightened, and all danger of oversetting is removed; for being
lighter without the balance weight than an equal bulk of water, it is
evident that the bell will rise as far as the length of rope affixed to
the balance weight will permit. This weight, therefore, will serve as a
kind of anchor, to keep the bell at any desired depth. Instead of wooden
seats, ropes are used suspended by hooks across the bottom of the bell;
and on these the diver stands. Two windows, made of strong thick glass,
are fixed near the top of the bell, G, H, two air-casks with their
tackle, and _c_, _c_, the flexible pipe, through which air is admitted
to the bell; each cask contains forty gallons. I, is a cock by which hot
air is discharged as often as it becomes troublesome, and a fresh supply
is obtained from the air-casks.
By another ingenious contrivance, Mr. Spalding rendered it possible for
the divers to raise the bell with all its weights to the surface, or to
stop at any particular depth, as they think proper; and thus they could
be safe, even though the rope for pulling up the bell was broken. This
was accomplished by affixing a second bell of smaller dimensions over
the large one as shown at K, K; it contains twenty-five gallons. In the
top of it is a cock, _d_, which can be opened by the diver to permit the
air to escape from the upper bell. Its handle comes down into the great
bell through the top at _d_. There is also another cock, _e_, in the
top, which permits the air to pass out of the great bell, and rise into
the small one. There is so much space left between the two bells, that
the water has free entrance into the upper as well as into the lower
bell. When the bell is first let down, the cock, _d_, in the top of the
upper one is opened, and therefore the air escapes from it, and the
water fills it. In this state the bell is lighter than an equal bulk of
water without the balance weight, though, with the addition of it, it is
heavier. Now if the divers wish to raise themselves, they turn the small
cock, _e_, by which a communication is made between the bells. The
consequence of this is, that a quantity of air from the lower
immediately rushes into the upper bell, and forces out a quantity of the
water contained there. The air which is thus let out from the lower bell
must be immediately replaced from the air-barrel, and thus renders the
bell lighter by the whole weight of water which is displaced. The air is
to be let out slowly, otherwise the bell will rise to the top with so
great violence, that the divers will be in danger of being shaken out of
their seats. The quantity let into the upper bell will determine the
rate of its ascent. Thus, if a certain quantity of air is admitted into
the upper cavity, the bell with the balance weight will descend very
slowly; if a greater quantity, it will neither ascend nor descend; and
if a larger quantity of air is still admitted, it will rise to the top.
[Illustration: Spalding’s improved Diving-Bell, with the Air-Barrels.]
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