522 HYDROMECHANICS [HYDRAULICS. a resistance lias to be overcome, the shaft N rotates relatively to P, compressing the springs, till their resistance balances the pressure due to the resist ance to the rotation of P. The engine then commences to work, the crank pin being in the posi tion in which the turning effort just overcomes the re sistance. If the resistance dimin ishes, the springs force out the chains and shorten the stroke of the plun gers, and vice versa. The following ex periments, on an engine of this kind working a hoist, show how the auto matic arrangement adjusted the water only 74 427 633 745 857 969 1081 1193 10 14 16 17 20 21 22 Fig. 177. used to the work done. The lift was 22 feet and the water pressure in the cylinders 80 lb per square inch. Weightlifted, ( Chain in pounds Water use !,in gallons 165. Accumulator Machinery. It has already been pointed out that it is in some cases convenient to use a steam engine to create an artificial head of water, which is afterwards employed in driving water-pressure machinery. Where power is required intermittently, for short periods, at a number of different points, as, for instance, in moving the cranes, lock gates, &c., of a dockyard, a separate steam engine and boiler at each point is very inconvenient ; nor can engines worked from a common boiler be used, because of the great loss of heat and the difficulties which arise out of condensation in the pipes. If a tank, into which water is continuously pumped, can be placed at a great ele vation, the water can then be used in hydraulic machinery in a very convenient way. Each hydraulic machine is put in communication with the tank by a pipe, and on opening a valve it commences work, using a quantity of water directly proportional to the work done. No attendance is required when the machine is not working. A site for such an elevated tank is, however, se^df T avail able, and in place of it a beautiful arrangement termed f accumulator, invented by Sir W. Armstrong, is used. This consists of a tall vertical cylinder; into this works a solid ram through cup leathers or hemp packing, and the ram is loaded by fixed weights, so that the pressure in the cylinder is 700 tb or 800 R> per square inch. The pumping engines which supply the energy that is stored in the accumulator should be a pair coupled at right angles, so as to start in any position. The engines pump into the accumulator cylinder till the ram is at the top of its stroke, when by a catch arrangement the engines are stopped. If the accumu lator ram descends, in consequence of water being taken to work machinery, the engines immediately recommence working. Pipes lead from the accumulator to each of the machines requiring to be driven. These pipes do not require to be of large size, as the pressure is so great. They are generally flanged pipes about 1-| inches bore, the joints being made by a gutta-percha ring. Fig. 178 shows in a diagrammatic way the scheme of a system of accumulator machinery. A is the accumulator, with its ram carry ing a cylindrical wrought-iron tank W, in which weights are placed to load the accumulator. At P>, is one of the pressure engines worked from the accumulator, discharging the water after use into the tank T. In this case the pressure engine is shown working a set of blocks, the fixed block being on the ram cylinder, the running block on the ram. The chain running over these blocks works a lift cage C, the speed of which is as many times greater than that of the ram as there are plies of chain on the block tackle. B is the balance weight of the cage. In the use of accumulators on shipboard for working gun gear or steering gear, the accumulator ram is loaded by springs, or by steam pressure acting on a piston much larger than the ram. Mr Tweddell has used accumulators with a pressure of 2000 Ib per square inch to work hy draulic riveting machinery. The amount of energy stored in the accumulator, having a ram d inches in dia meter, a stroke of S feet, and work ing at p pounds pressure per square inch, is JTpcPS foot-pounds. 4 Thus, if the ram ~ is 9 inches, the stroke 20 feet, and the pressure 800 K> per square inch, the work stored -, in the accumula- tor when the ram is at the top of the stroke is 1,017,600 foot- Fig _ 178 pounds, that is, enough to drive a machine requiring one horse power for about half an hour. As, however, the pumping engine replaces water as soon as it is drawn off, the working capacity of the accumulator is very much greater than this. m Fig. 179. Water Wheels. 1 66. Overshot and High Breast Wheels. When a water
fall ranges between 10 and 70 feet, and the water supply
