Chap. XIII.
1051
same time the other descended, thus by alternate succession furnishing air so quick and insuch plenty, that I myself have been one of five who have been together at the bottom innine or ten fathoms water, for above an hour and a half at a time without any sort of illconsequence ; and I might have continued there as long as I pleased, for any thing thatappeared to the contrary. Besides, the whole cavity of the bell was kept entirely freefrom water, so that I sat on a bench, which was diametrically placed near the bottom, withall my clothes on ; I only observed that it was necessary to be let down gradually at first,or about 12 feet at a time, and then to stop and drive out the water that entered, by re-ceiving three or four barrels of fresh air before descending any further, but being arrivedat the depth designed, I then let out as much hot air that had been breathed as each barrelwould replenish with fresh air, by means of the cock at the top of the bell, through whoseaperture, though very small, the air would rush with so great violence, as to make thesurface of the sea boil, and cover it with a white foam, notwithstanding the great weight ofwater over us.”
Since Dr. Halley's invention, many improvements have been made in the diving bell,particularly by Mr. Smeaton , who seems to have been the first engineer that used it forsub-marine constructions.
When the diving bell is employed in still water, it is usually suspended between twoboats of 15 or 20 tons each, over the decks of which is laid a platform of timber, to keepthem at the proper distance from each other, and on which the tackle is worked for loweringand raising the bell.
Smeaton employed the diving bell as early as 1778, in the constructions of the bridge atHexham : in a letter which he addressed to Mr. Pickernell on the subject, we have themethod he adopted fully explained : he says, “ If the cases would have enabled us toreduce the water so low, as to be even with the very bottoms of the caissoons of each pier,I take it for granted, you would have thought it no difficulty with broken rubble, beton,stones, and short blocks of wood, cut a little wedgeways, to have crammed and wedgedup the cavity washed under the wooden bottoms, so as to have been equally resisting,and capable of bearing a weight with the original gravel, and particularly when thisnew body of matter is supported, and even jammed tighter into its place by filling upthe vacancy between the pier and the base, a little above the wooden bottom, with rubble,and then driving it tight down by a set with the ram. It therefore now remains that 1describe, and make you master of a piece of machinery, that will put you nearly into thesame condition, as if the water could have been reduced to the caissoon’s bottoms as beforementioned ; and this is by means of an air-chest, or diving vessel, which being let down,will exclude the water down to the very bottom of the diver if you please, and therefore aslow as the under side of the wooden bottom, which in the present case is as low as will benecessary or useful, and the chest or vessel being large enough to give liberty for a man towork therein, being furnished with a pair of boots, he will at mid leg deep in water dohis business with almost as much facility as if the water were pumped out to the same level.The principal part of this machine will consist of a strong chest, suppose d feet 6 inches inlength, about 4J feet deep or height, and as wide as to give free leave for its going downbetween the cases and the piers, which I suppose will be about 2 feet wide inside measure,as the other measures are also supposed to be. Now you know very well that if you pusha drinking glass, or any other similar vessel, with its mouth downwards into the water, thatit will exclude the water, leaving the vessel full of air, as it was before it was thrust intothe water ; in like manner, if this chest, being loaded with a sufficient weight, be let downinto the water, mouth downwards, the air will exclude the water to the bottom skirt of thechest, and if let down, so as to rest upon the bottom of the river, a man may stand therein,and do any kind of business, the same as he could do in the same space in the open air.But to continue this for any length of time two things are obviously necessary, and thoseare light and a circulation of fresh air. The former might on occasion be supplied by acandle, but here we may have the advantage of day-light by putting two or three stronground panes of glass into the bottom of the chest, which will in its inverted situation be thetop; a sufficiency of light will enter, this top of the chest being supposed above water.Respecting air, you will conceive that any quantity might be forced in by a strong pair ofbellows ; but those made of leather would be cumbersome and unhandy: I therefore sub-stitute a kind of foreign air-pump, made of thin hammered copper, that will throw in agallon at a stroke, which will not only continually refresh the workmen within, but what-ever air escapes through the joints or pores of the air-chest will be replenished, and theoverplus go out at the bottom or skirt of the chest, and boil up on the outside.
The quantity of weight that will sink it, mouth downwards, will be the same as placedtherein (bottom downward) would sink it the same depth ; and as this chest I propose tobe suspended by a tackle, and to go down by its own weight, I compute that it will takesixteen pigs of lead to sink it to the bottom of the river, and keep it steady. I proposethat the lead may be as much out of the way as possible, to place them upon the ends of