10 5 l 2
THEORY AND PRACTICE OF ENGINEERING.
Book IT.
the chest, endways upward, that is four in a row below and four above, and the same at theother end, making in the whole 16 pigs, which are to be fastened on with screws, cither bycleats screwed on, or punching a hole through each end of each pig. At one end of thechest there is to be a board, fixed across for the man to sit upon, and a cleat nailed to eachside to set each of his feet upon, so that while the machine is being lowered or hoisted, he istotally dry, and when let down enough, he stands upon the bottom of the river, without anymore water than the height between the skirt of the chest and the bottom of the river, whichmay be more or less as is found convenient, I suppose never more than a foot deep, becausewherever the ground is taken out more than 1 foot below the underside of the caissoon'sbottom, I would propose to fill it up with rubble previously to that height or depth ; norcan it be of use to let down the skirt of the chest much below the caissoon’s bottom, becausethe side of the chest will then diminish the room you will have to get the matter for under-pinning under the caissoon’s bottom. The foregoing will, I believe, be sufficient for ex-plaining the general principles and outlines of the method I mean to pursue in under-pinning, and re-supplying what is underwashed from the bases of the piers, and which Idare say you will now see to be entirely practicable : what you are therefore immediatelyto put in hand is the air-chest, of or about the inside dimensions before mentioned; I believethe two flat sides will do very well, if of good red wood deal, shot clean of sap, the twoends and bottom, (or in use its top) ; it would be well if they could be got of single planksof elm, beech, or plane trees, as they would hold the nails better : I fancy IJ or ij inchesthick for the sides, 2\ or 2^ for the ends and bottom, will be sufficient; they should be welljointed, and put together with white lead and oil, as the effort will not be of the water toenter, hut of the air to escape from within. Were I with you, when it is put in use, Ishould be the first to go down into it, as there is no more danger (all your tackle beingfirmly fixed) than being let down into a coal-pit by a rope : and if it shall happen that allyour masons are too fine fingered, I fancy a couple of colliers to take turn and turn willfind it a very comfortable job. A particular encouragement, must, however, I expect begiven. I will give you more particular directions in my next: as to the air-pump, allthat will be wanted from the coppersmith will be a cylindrical pipe of copper, 10 inchesdiameter, and 12 inches high, wired at top, and a flanch at bottom of about 1J inches broad,by which it is screwed down before the top of the air-chest: the copper to be about thethickness of a halfpenny; if you have noneat-handed coppersmith that can hammerit straight and smooth inside, it may on oc-casion be made of strong tin.”
“ A, the air-pump, B. sky-lights, 6 inchesin diameter, to be made of window gla:>sknobs, if plate glass cannot be had ; C,clamp plates of iron, to confine the top andsides strongly together ; D, I), pigs of lead,end upwards ; E E, the lever for workingthe pump; G G, the axis and brace for steady-ing the lever; II. II. two bows for hoistingthe chest; I, a strong hooked iron to layhold of the bows, to which the main rope ortackle is to be fixed : M N, the opening fromthe pump to the air-chest: op, the valve,whereof o is leather, p wood, to be shut bya wire spring qrs, a little more than suffi-cient to overcome the weight of the valve.”
The diving bell at Her Majesty’s dockyardat Plymouth is of cast-iron, and weighs 4tons 2 cwt. It is 6 feet long, 4 feet broad,and 5 feet in height; its cubical contentbeing 120 feet.
Twelve convex lenses, each 8 inches indiameter, are placed at the top to admit thelight, which when sunk in clear water issufficient to enable the diver to read thesmallest print. In the centre of the top is ahole for the admission of air, which is sup-plied by a leathern hose long enough toreach any moderate depth ; at its upper endis attached a forcing or air-pump, which isworked by four men during the time thebell is immersed, and the quantity of air supplied enables those in the bell to respire with