Part 23 (1/2)
It is a characteristic of fluids and gases that if pressure be brought to bear on any part of a mass of either class of bodies it is transmitted equally and undiminished in all directions, and acts with the sales to those surfaces
The great natural philosopher Pascal first formulated this reiven in Fig 179 Two cylinders, A and B, having a bore of one and two inches respectively, are connected by a pipe Water is poured in, and pistons fitting the cylinders accurately and of equal weight are inserted On piston B is placed a load of 10 lbs To prevent A rising above the level of B, it must be loaded proportionately The area of piston A is four tiht, neither piston willpipe are also pressed outwards in the ratio of 10 lbs for every part of their interior surface which has an area equal to that of piston B
[Illustration: FIG 179]
[Illustration: FIG 180--The cylinder and ram of a hydraulic press]
The hydraulic press is an application of this law Cylinder B is represented by a force puh pressures (up to 10 tons per square inch) In the place of A we have a stout cylinder with a solid plunger, P (Fig 180), carrying the _table_ on which the object to be pressed is placed Brareat difficulty in preventing the escape of water between the top of the cylinder and the plunger If a ”gland” packing of the type found in steam-cylinders were used, it failed to hold back the water unless it were screwed down so tightly as to jaer He tried all kinds of expedients without success; and his invention, excellent though it was in principle, seemed doomed to failure, when his foreman, Henry Maudslay,[35] solved the problem in a simple but most masterly manner He had a recess turned in the neck of the cylinder at the point for-box, and into this a leather collar of U-section ( 180) was placed with its open side doards When water reached it, it forced the edges apart, one against the plunger, the other against the walls of the recess, with a degree of tightness proportionate to the pressure On water being released froer to sink without friction
The principle of the hydraulic press is e, and riveting steel plates, or forcing wheels on or off their axles; for advancing the ”boring shi+eld” of a tunnel; and for other purposes too numerous tothese, the most used is the tap, or cock When a house is served by the town or district water supply, the fitting of proper taps on all pipes connected with the supply is stipulated for by the water-works authorities The old-fashi+oned ”plug” tap is unsuitable for controlling high-pressure water on account of the suddenness hich it checks the flow Lest the reader should have doubts as to the nature of a plug tap, wein a tapering socket On the cone being turned till a hole through it is brought into line with the channel of the tap, water passes A quarter turn closes the tap
[Illustration: FIG 181--A screater cock]
Its place has been taken by the scren cock A very co 181 The valve V, with a facing of rubber, leather, or some other sufficiently elastic substance, is attached to a pin, C, which projects upwards into the spindle A of the tap This spindle has a screw thread on it engaging with a collar, B
When the spindle is turned it rises or falls, allowing the valve to leave its seating, V S, or forcing it down on to it A packing P in the neck of B prevents the passage of water round the spindle To open or close the tap completely is a h to produce a ”water-ha the flow The reader will easily understand that if water flowing at the rate of several miles an hour is abruptly checked, the shock to the pipes carrying it must be very severe
THE BALL-cock
is used to feed a cistern auto too far in the cistern (Fig 182) Water enters the cistern through a valve, which is opened and closed by a plug faced with rubber
The lower extreular hole cut in it Through this passes a lever, L, attached at one end to a hollow copper sphere, and pivoted at the other on the valve casing This casing is not quite circular in section, for two slots are cast in the circu freely when the valve is open The buoyancy of the copper sphere is sufficient to force the plug's face up towards its seating as the valve rises, and to cut off the supply entirely when a certain level has been attained If water is drawn off, the sphere sinks, the valve opens, and the loss is ood
[Illustration: FIG 182--An automatic ball-valve]
THE WATER-METER
[Illustration: FIG 183]
Soe of a water supply, and the water company allows them to use as much as they require
Others, however, prefer to pay a fixed aallons used In such cases, a water-meter is required to record the consura 183), very widely used At the botto cylinder, fitted with a piston, (6), which is ht and free fro of india-rubber, rolling between the body of the piston and the internal surface of the cylinder The piston rod (25), after passing through a stuffing-box in the cylinder cover, is attached to a rack, (15), which gears with a cog, (13), fixed on a shaft As the pistonis turned first in one direction, then in the other To this shaft is connected the index ht) The cock-key (24) is so constructed that it can put either end of thecylinder in coiven a quarter turn (see Fig 184) The weighted lever (14) h part of a circle From the pinion project two arms, one on each side of the lever When the lever has been lifted by one of these past the vertical position, it falls by its oeight on to a buffer-box rest, (18) In doing so, it strikes a projection on the duplex lever (19), which is joined to the cock-key, and gives the latter a quarter turn
In order to follow the working of the s 183 and 184 si froh the cock doards through channel D into the lower half of the cylinder The piston rises, driving out the water above it through C to the delivery pipe B Just as the piston coht, raised by the rack and pinion, topples over, and strikes the key-arm, which it sends down till stopped by the buffer-box The tap is then at right angles to the position shown in Fig 184, and water is directed fro the piston dohile the water ade D, and out by the outlet B
Before the piston has arrived at the bottohted lever from the buffer-box, and raised it to a vertical position; froht-hand key-arht the cock-key to its forin another upward stroke
[Illustration: FIG 184]
The _index mechanism_ makes allowance for the fact that the bevel-wheel on the pinion shaft has its direction reversed at the beginning of every stroke of the piston This bevel engages with two others mounted loosely on the little shaft, on which is turned a screw thread to revolve the index counter wheels Each of these latter bevels actuates the shaft through a ratchet; but while one turns the shaft when rotating in a clockwise direction only, the other engages it whenan anti-clockwise revolution The result is that the shaft is always turned in the same direction
WATER-SUPPLY SYSTEMS
The water for a town or a district supply is got either from wells or from a river In the former case it may be assumed to be free fro all the objectionable and dangerous reater or less degree This purification is acco tanks_, where the suspended matter sinks to the bottom The water is then drawn off into _filtration beds_,manner The bottom is covered with a thick layer of concrete On this are laid parallel rows of bricks, the rows a small distance apart Then coether; a layer of coarse gravel; a layer of finer gravel; and a thick layer of sand at the top The sand arrests any solid ravel and drains below