1544

THEORY AND PRACTICE OF ENGINEERING. Book II.

liable to chip, the joints wear away, and allow the water to pass through, which it is exceedinglydifficult to prevent. These inconveniences may be in great measure avoided by placingin front of the stone sills two pieces of timber, to receive the gates when closed, which canbe much more easily cut than the stone, and consequently the gates, &c., be made to fitexactly ; by bolting the pieces of timber to the stones beneath them, and securing them inthe wall, and filling the joints with moss, all filtration between the timber and the stone maybe effectually stopped.

In locks of rivers whose course is over stones or gravel, there is considerable inconveniencewith regard to the pointed sills, pebbles often lodging between them and the gates, prevent-ing their close shutting, andcausing a loss of water,which it is very difficultentirely to prevent, butwhich is in some degreeobviated by a space of a fewinches being left betweenthe gate and the bottom ofthe platform; the sill andthe lower rail might alsobe chamfered, so that thegate in shutting shoulddrive the pebbles before it.

Sluices to gates are about2inches square, and areraised above the lower railof the frame, so that thewater which comes outmakes over the gate downstream a fall over the plat-form, which wears away,and in the course of timematerially injures it. Itwould therefore be moreadvantageous to make thewater flow out under thegates, and throughout theirwhole width ; there wouldthen be only a film, whichacting horizontally wouldbe less injurious than whenflowing in a direction in-clined to the horizon.

The lower rail should

also be placed at 12£ inches above the bottom of the posts; the sluice should only be asimple plank, equal in length to the entire width of the gates, with its ends attached to twosmall posts raised by means of a lever. Thepointed sill, against which this rests, should havebut little height, and the bottom of this sluice-board should be cut in a bevel form, to throw offthe stones and gravel that might be driven towardsthe sill, which it is sufficient to make more elevatedin the centre to receive the chamfered or bevelledplanks forming the sluice.

This construction has another great advantage,that of producing a current under the gates,carrying away whatever might stop there, and isparticularly calculated for river gates.

It is generally admitted that the portion of theplatform where the gates move should not be in aninverted arch, but extremely solid, and so constructedthat the water cannot filtrate through or flow out atthe platform between the discharging shoulders: ifattached to the pointed sill, which must be alwayson a level, the form of an inverted arch may begiven to it.

The Gates of Lochs are composed of two postsplaced vertically, and several horizontal rails; the

Fig. 2849.

SLUICES TO LOCK-GATES.

r ^ 1

Fig. 2800. DOUBLE VALVE FOR TIDE.