D IV

170

D IV

unless they be dilated 'and distended by a considerable force.Hence, on the first descent of the bell, a pressure begins to be felton the ear; which, by degrees, grows painful, till the force over-coming the obstacle, what constringes these pores yields to thepressure, and letting some condensed air slip in, presently easeensues. The bell descending lower, the pain is renewed, andagain eased in the same manner. But the greatest inconvenienceof this engine is, that the water entering it, contracts the bulk of airinto so small a compass, that it soon heats and becomes unfit forrespiration, so that there is a necessity for its being drawn up torecruit it; besides the uncomfortable situation of the diver almostcovered with water. To obviate these difficulties, Dr. Halleycontrived some further apparatus, whereby not only to recruit andrefresh the air from time to time, but also to keep the water whollyout of it at any depth. The bell he made use of was of wood, con-taining about 60 cubic feet in its concavity; and was of the formof a truncated cone, whose diameter at the top was three feet, andat the bottom five. This he coated with lead so heavy that it wouldsink empty; and he distributed the weight so about its bottom,that it would go down in a perpendicular direction, and no other.In the top was fixed a strong clear glass, as a window, to let in thelight from above; and likewise a cock to let out the hot air that hadbeen breathed : and below, about a yard under the bell, was placeda stage which hung by three ropes, each of which was chargedwith about 100 weight to keep it steady. This machine he sus-pended from the mast of a ship by a sprit, which was sufficientlysecured by stays to the mast head, and was directed by braces tocarry it overboard clear of the ship’s side, and to bring it againwithin board as occasion required. To supply air when underwater he caused a couple of barrels of about 36 gallons each to becased with lead, so as to sink empty ; each of them having a bung-hole in its lowest part to let in the water, as the air in them con-densed on their descent; and to let it out again when they weredrawn up full from below. To a hole in the uppermost part ofthese barrels, was fixed a leathern trunk or hose well liquored withbees wax and oil, and long rnough to fall below the bung-hole,being kepi down by a weight appended: so that the air in theupper part of the barrels could not escape, unless the lower cudsof these hose were first lifted up. The air-barrels being thus pre-pared, he fitted them «ith tackle proper to make them rise andfall alternately, after the manner of two buckets in a well; whichwas done with so much ease, that two men, with less than halttheir strength, could perform all the labour required : and in theirdescent they were directed by lines fastened to the under edge oIthe bell, which passed through rings on both sides the leathern hosein each barrel; so that, sliding down by these lines, they camereadily to the hand of a man who stood on the stage on purposeto receive them, and to take up the ends of the hose into the hell.Through these hose, as soon as their ends came above the surfaceof flic water in the Ix.rivls, all the air that was included in the upperparts of them was blown with great force into the bell; wh ist tin-water entered at the bung-holes below, and filled them: and assoon as the air of one barrel had been thus received, upon a signalgiven, that was diawn up, and at the sametime the other descended;and, by an alternate succession, furnished air so quick, and in sogreat plenty, that he and four others were together at the bottomin nine or ten fathom w ater, for above an hour and an half at atime, without any ill consequence: and might have continued thereas long as they pleased, for any thing that appeared to the contrary.Til descending they found it necessary to he let down gradual y atfirst, as about 12 leet at a time ; and then to stop and drive out theair that entered, by receiving three or four barrels of fresh air.But being arrived at the depth designed, as much of the hot airthat hail been breathed, as each barrel would replenish with co..l,was let out by means of the cock at the top of the bell; throughwhose aperture, though very small, the air would rush with somuch violence, as to make the surface of the sea boil, and tocover it with a while foam, notwithstanding the weight of thewater above. And thus by the introduction of air a space at thebottom of the sea as wide as the circuit of the bell, could be madeso far dry, as not to be overshoes thereon. And, bv the glasswindow, so much light was transmitted, that when the sea wasclear, and espocall) w hen the sun shone, they could l ee perfect-ly well to write or read ; much more to fasten or la) hold on anything that was to be taken up. By an additional contrivance, hefound it not impracticable for a diver to go out of an engine to a

good distance from it, the air being conveyed to him with a con-tinued stream, by small flexible pipes; which pipes may srrveas a clue, to direct him back again when he would return to thebell. Plate L1X. fig. 1, shews Dr. Halley’s aiving-bell, with thedivers at work. DBLKRI M P represents the body of the bell.D, the glass which serves as a window. B, the cock for lettingout the air which has been breathed. I,M, the seats. C, one ofthe air-barrels. P, H, two of the divers. F, another diver at adistance from the bell, and breathing through the flexible tube K.'t his diver is supposed to have a head-piece of lead, made to tilquite close about his shoulders: this head-piece was capable ofcontaining as much air as would supply him for a minute or two.When he had occasion for more air, he turned a cock at F, bywhich means a communication was opened with the air in the bell,and thus he could receive a new supply at pleasure.

Diving-Bell, Mr. Triewald ’s.’ Mr. Triewakl, F. R. S.and military architect to the late king of Sweden , has invented adiving-bell, (fig. 2,) which, for a single person, is thought to bemore eligible than Dr. Halley’s, and is constructed as follows:AB is the bell, which is sunk by lead weights DD hung to itsbottom. 'Phis bell is of copper, tinned ail over in the in-side, which is illuminated by three strong convex lenses,G, G, G, with copper lids H, if, H, to defend them. The ironplate E, serves the diver to stand on when he is at work ; and issuspended at such a distance from the bottom of the bell by thechains F, F, F, that when the diver stands upright, his head isjust above the water in the bell, where the air is much better thanhigher up, because it is colder, and consequently more fit for re-spiration. But as the diver must always be within the bell, andhis head of course in the upper part, the inventor has contrived,that even there, when he has breathed the hot air as long as hecan, he may, by means of a spiral copper tube b c, placed close tothe inside of the bell, draw the cooler and fresher air from thelowermost parts: for which purpose, a flexible leather tube, abouttwo feet long, is fixed to the upper end of the copper tube at b ;and to the other end of this tube is fixed an ivory mouth-piece, bywhich the diver draws in the air.

Diving-Bell , Spalding’s improved. The greatest im-provement, which the diving-bell ever received, was from the lateSir. Spalding of Edinburgh . A section of his improved diving-bell is represented in fig. 3. This construction is designed to re-medy some inconveniences of Dr. Halley’s, which are very evi-dent, and of very dangerous tendency. These are, 1. By Dr.Halley’s construction, the sinking or raising of the bell dependsentirely on the people who are at the surface of the water; and asthe hell even when in the water has a veiy considerable wi ight, theraising it not only requires a great deal of labour, but there is apossibility of (lie rope breaking by which it is raised, and thusevery person in the fell would inevitably perish. 2. As there are,in many places of the sea, rocks which lie at a considerable depth,the ligure of which cannot possibly be perceived from above,there is danger that some of their ragged prominences may catchhold of one of the edges of the bell in its descent, and thus over-set it before any signal can he given to those above, which wouldiniallibly be attended with the destruction of the people in thebell: and as it must always be unknown, before trial, wliat kindof a bottom the sea has in any place, it is plain, that without somecontrivance to obviate thi> last danger, the descent in Dr. Halley’sdiving-bell is not at all eligible. How these inconveniences are re-medied by Mr. Spalding’s new construction will be easily under-stood from the following description.—ABCD represents a sec-tion of the bell, which is madi ot wood ; e, e, are iron liook«, bymeans of which it is suspended by ropes QBEe, andQAEUe,and QS, as expressed in the (Lure : c, c, arc iron hooks, to whichare appended lead weights, that keep the mouth of the bell alwaysparallel to the surface ol the water, whether the machine taken al-together is lighter or heavier than an equal hulk of water. Bvthese weights alone, however, the I ell would not sink another h.therefore added, represented at 1.; and which can he raised orlowered at pleasure, by means ol a rope passing ov> r the pulk j a,and fastened to out ol the sides ol the bell at M. As the bell de-scends, this weight, called by Mr. Spa'll ng the balance-weight,Hangs down a considerable wav In lew the mouth oi the hell. " Incase the edge ol the bell is call lied l \ an\ c.b-t-cle, tin balance-weight is immediately haver, d down -o ih. I a may ie-t upon thebottom, By this means tin bul is lightened so that all danger of•i oversell mg