The experienced reader will See, that this pump is very like that of Gosset and De la Deuille, described by Belidor, vol. ii. p. 130, and most writers on hydraulics. It would he still more like it, if the bag were on the under side of the partition E, and placed farther down the pump; but we think that our form is greatly preferable in point of strength. When in the other situation, the column of water lifted by the piAon tends to burst the bag, and that with a great force, as the intelligent reader well knows. But, in the form recommended here, the bag is compressed, and the strain on each part may be made much less than that which tends to burst a bag of 6 inches diameter. The nearer the rings are placed to each other, the smaller will the strain be.
"The same bag piston may he employed for a forcing pump, by placing it below the partition, and inverting the valve ; and it will then be equally strong, because the resistance in this case too will act by compression." A double pump, of a nature similar to that which has no% been explained, has been described by Hachette, in his Traite Elementaire des Machines, p. 153.
Those who are acquainted with the fine manufacture of water-proof cloth and canvass by Mr. Charles Mack intosh of Glasgow, will see that pump of the above de scription, with bag pistons, may be constructed more ele gantly and durably by using those water-proof fabrics made by a varnish obtained from the dissolution of caoutchouc, in the naptha procured from coal tar.
9. Description of an Occasional Pump, of a simple construction.
This pump, which is represented in plate CCCCLXX. Fig. 13, was suggested to Dr. Robinson while describing the construction of a simple valve. In a square wooden truk ABCD, a piece of oak board EF is exactly fitted to the trunk in an oblique position, and supported by an iron pin, which goes through it at I, one-third of its length from the lower extremity E. The two ends of the board EF are bevelled so as to apply themselves accurately to the • sides of the trunk. If a stream of water now rises upwards, it will press with more force on the part IF of the board, than upon the part El, and consequently it will force it up, and rush through, causing the board to stand nearly paral lel to the sides of the trunk. In order to prevent it from rising into the parallel position, its pro•ess must be stop ped by a projecting pin. If the streannof water now de
scends, its pressure on the upper side of the board being again greatest on the part IF, it will be forced back again into its former position, and its two bevelled extremities resting on the opposite sides of the trunk, the passage will be completely shut up. The board EF will therefore per form the office of a very perfect valve, both because it affords the freest passage for the water, and allows very little to get back while it is shutting; for the part IE brings up half as much water as IF allows to go down. The tightness of this valve may be greatly increased by fixing two thin fillets G and H to the sides of the trunk, and covering with leather those 'parts of the board EF, which come in contact with them.
The valve being thus constructed, a square box, abcde, covered on the outside with soft leather, is made to slide, without sticking, along the wooden trunk ABCD, and a piston rod is fixed to a piece of wood e, morticed into two of the sides of the box a b c d e, which project upwards like the gable ends of a house. A valve similar to EF is placed in this box, and it becomes a pump of the usual form. Dr. Robison remarks, that, if this pump is mersed so deep in the water that the piston shall also be under water, its performance will be equal to any pump.
10. Description of Delahire's Double Forcing Pump.
This pump is represented in plate CCCCLXX. Fig. 14, and partakes both of the nature of a Totting and a sucking pump. It consists of the great barrel AB, to which is connected the rising pipe EF, with valves opposite to E and F, and the main pipe CD, with valves opposite to C and D. The piston b is of one piece, without any valve, and the piston rod a b works in a collar of leather at A. When the piston is depressed to B, the valve F will shut, and the air below the piston will be driven through C up the pipe CD ; and, in consequence of the rarefaction of the air above the piston, the valve D will shut, and the water will rise up FE, through the valve D, into the barrel above tre piston. When the piston is raised towards A, it will force up the water above it, through the valve D, and up the pipe DC, while water will rise through the pipe HF, and pass through the valve at F, into the barrel below the piston.
11. Description of a Centrifugal Pump.