1210

THEORY AND PRACTICE OF ENGINEERING

Rook II.

times lodges in the upper portions, and stops the water’s motion ; air-holes and stop-cocksmay be used to prevent this, which can be shut when the water is in its proper condition.

1896 .

If

1 * 97 .

The forms of cocks usually employed in water-pipes are represented in the several figures, with threebranches at right angles to each other; the tube ispierced in such a manner as to shut off or supply thebranches together or alternately; the cocks mustalways be proportioned to the size of their pipes, andwhen too large to be turned by hand must be workedby an iron key and lever.

Fig. 1900.

ciiap. xvm.

SUPPLY OF TOWNS WITH WATER.

We have already described the methods practised by the Romans for the supply of theirtowns with water, and also the attention paid to the subject at a very early period in our owncapital; but that much yet remains to be done to ensure cleanliness, and consequentlyhealth, has been recently proved in the excellent report made “ upon the State of Townsand Populous Districts,” by Her Majesty’s Commissioners: with improved machineryand the aid of the steam-engine, which enables us to raise water with perfect ease and at asmall cost to almost any height, we may hope ere long to see the object of the Com-missioners fully carried out, in obtaining a plentiful supply of wholesome water for everydwelling throughout the empire. The engineering works for such a purpose would beneither difficult of execution, nor intricate in their arrangement.

The quantity of water given out by a pipe under pressure, when the aperture and head ofwater are contiguous, is easily calculated; hut when the water is to be delivered at a dis-tance, the amount of friction and resistance of the fluid in its passage must be taken intoaccount: practical experience has established many rules to guide us on this subject, whichhave been generally applied by most of the engineers of our water companies: when a town isso situated that water can be brought from a higher source than where it is required tobe delivered, there is no difficulty whatever in regulating its descent; a reservoir must be con-structed of sufficient dimensions to contain the ordinary supply, from which the mains arelaid, with their branch pipes to the houses or other situations where the water is to bereceived.

The mains may be kept constantly full by the aid of a cock at the point of delivery:where the water is to be shut eff from one part of the town whilst it is running to another,each house must have a separate cistern with a ball-cock, which shuts off'the water when itarrives at a certain height, and prevents any waste. To raise the water from a low level toa higher, some additional works are requisite : a steam-engine is employed to throw itto a reservoir upon some high ground, from whence sufficient pressure may be gained todeliver the water to its destination; and when it is to be sent to any point above the levelof the highest reservoir, a simple and ingenious contrivance has been made use of, previouslyto the introduction of which it was found that when the aperture leading to the reservoirwas closed, and the engine continued to force the water forward, the excess of pressureburst the pipes. It consists of two upright standing pipes united at the top, and forming asiphon, above which is another upright pipe open at the top, standing up twenty or thirtyfeet above the reservoir : as this apparatus is connected with the upper supply, as soon asthe pressure exercised is greater than that arising from the head, the water will flow out ofhe top of the pipe into the reservoir below, thus forming a safety-valve.