1214

THEORY AND PRACTICE OF ENGINEERING.

Book It.

Sluice Corks , for letting off the water of a reservoir intothe mains, are of various kinds ; the most ingenious arethose by which the valve can be opened and shut at

pleasure by means of levers.The valve which covers themouth of the dischargingpipe has two arms securedto an iron axle, at the ex-tremities of which are twolevers; the chain fastenedto one passes under, and theother over, a pulley, so thatby turning a windlass eitherchain can be worked for thepurpose of opening or shut-ting the valve.

The Screw is applied tothe same purpose, that ofshutting oir the flow of waterin the mains.

Lnyinq down the Mains.The general rule according

to Mr. T. Wicksteed to befollowed, is “to take tare thatthe distances between the

mains shall not be too great,for if it be so it will be neces-sary to have services of largerdiameter than would other-wise be required : to preventloss in pressure occasionedby friction, the communi-cations between the mainsand services must be keptopen for a longer time, togive a supply to those housesat the extremity of the ser-vice ; for w T hen water is forcedthrough pipes, either by anatural or artificial head, orby steam or other power,friction is created in pro-portion to the velocity of

SCREW COCK