1184

THEORY AND PRACTICE OF ENGINEERING.

Book II.

usual. This valve, placed at the Iwttom of the branch, is lodged in a drum I K, so as notto contract the passage of the water at that place ; the lower part of the brancli is swelled,in order to have a ring, Y', all round the flange of the pipe LM for the valve to rest upon,which is consequently horizontal, a situation preferable to the vertical, which do not closewell; it is true the void space is not so great.

This valve must be made strong, otherwise the leather hinge will soon be injured ; beingthe weakest part, these kind of valves always give there, especially when much strained, andbeing liable to frequent reparations are consequently not convenient for large pipes, as infalling they often slip more on one side than another and do not close exactly; the leatherof the hinge becoming too flexible has not sufficient bodyto keep the valve in its proper direction. To remedy thisinconvenience, when the branch is 8 or 10 inches diameter,a valve may be formed of two clacks, consisting of a ringof copper, as in the first piston described, whose lesser dia-meter is equal to that of the ascending pipe, and its greater a \sufficient to admit of being held between the flanges ofthe drum and those of the curved pipe. This ring shouldhave a bar, D, so as to make one piece of the whole, asshown in section in fig. 1811., where L M represents thisring with the bar A ; a leather circle, 2 inches more indiameter than the ascending branch, will form the twoclappers, E, I, strengthened above and below by copper platesG,H. The circle of leather is laid on the cross piece, A; J -a copper rule of the same length is then placed above it, andthey are united by two screws, as in fig. 1810., where the ruleB C has a knob D E to prevent the two claps both falling onthe same side. This valve is the most convenient, and

Fig. 1810

Fig. 1811.

contracts the water-way so little as not to be of any importance.

Leather clappers not being very durable, those of copper are substituted in two semi-circles, united by a hinge, as in figs. 1812, 1813, 1814, 1815. The lower, on the righthand, represents themwhen closed ; they are fittedin a box B B, projectingequally with them, andmovingontwohinges, G, G,whose axis enters the twouprights E : on the top ofeach is a button F, to main- A !tain the clappers in thesituation shown by the twoupper figures, and obligeeach to fall towards its ownside, when the piston ceasesto force. The tongue A Ashould be formed withroundles of leather be-tween the flanges, IK and aL M.

In selecting the bestform for valves, we mustalways consider the pur-pose to which they are tobe applied ; the most im-portant considerations are,that it should be perfectlytight; that it have sufficient strength to resist the forces to which it is exposed ; that it allow’the water to pass freely, and not permit it again to flow hack, whilst it is in the act of closing.The clack valve, which is the simplest, should be fixed in a box like a piston ; its outer sur-face should be a little conical, so as to fit a similar recess made for it in the tube; it shouldhave an iron ring or handle to enable it to be taken out by a hook whenever it is required tobe drawn up . the top which the clack valve sustains, when opened, may be considered asequal to one-half of the cylinder of water, whose height is equal to the diameter of thevalve. The spherical valve obstructs too much the passage of the water, and when madevery light there is the probability that it will not return to its place when required, withoutallowing a quantity of water to flow’ back.

Another valve placed at the bottom of reservoirs or basins for the purpose of layingthem by or filling them, fig. 1816.; it consists of a copper box A B C 1), having a rebate B C

Fig. 1814

Fig. 1812.

V

L

M

Fig. 1813.

Fig. 1815.