1548
THEORY AND PRACTICE OF ENGINEERING.
Book II.
Fig. 2862.
FRESHWATER LOCK.
sixth of the span, or a little more:these proportions not only allowof the gates being smaller, lighter,and stronger, but also increasethe pressure of the heel-post againstthe hollow quoins, which rendersthem quite water-tight. Wherecanals are narrow, the paddles ofboth the upper and lower gates areusually kept open by an iron pininserted between the teeth of arack and pinion which raises them :when the paddle is required to beshut, the pin is withdrawn, and thepaddle falls by its own weight.
Hollow QuoinSy or upright circulargrooves, are formed in the sidewalls, at the ends of the timbersills, serving as the hinge for thegates; the upright post that turnswithin them is called the heel ofthe gate, and the other the head.
The former are retained in theirposition by a gudgeon or pivotturning in a cup let into the found-ation stones for the purpose; some-times the pivot is fixed, and thecup revolves upon it.
The upper part of thepost is retained by aniron ring or strap letinto the side wall, andmade very secure ; thehollow quoins shouldbe worked with greatattention; they areusually of stone orbrick, though cast-ironhas been found wellsuited for the purpose.
Dock Gates of largeDimensions are nowusually opened and shutby machinery, and theboom or spar attachedto the head-post en-tirely dispensed with:on many canals a rack-bar of wrought-iron isconnected with thegates, which are fur-nished with rollers torun in a groove fittedinto the sill, and byworking a wheel andpinion, they can beopened and shut at
pleasure. We ought not to omit mention of several gates formed like boats, upon theprinciple of the camel, which rise and fall in deep recesses prepared to receive them aswater is pumped out or admitted into them: such boat-gates are sometimes constructedwith three parallel keels, which fit into as many grooves in the side walls of the lock ; theyare maintained in their position by admitting the water, and raised by pumping out theircontents, after which they are floated away ; for the stop-gates of docks such a contrivanceis well adapted, but where the navigation is regular, as on a canal, they are not found toanswer, from the time requisite to open and replace them.
The Boom or Spar attached to the head-posts is brought over a windlass, placed on thelock walls a little above the gate; a rope is made fast to the end of the spar, and by two or