Chap. XXVII.

CANALS AND RIVER LOCKS.

1533

CHAP. XXVII.

CANALS AND RIVER LOCKS.

Canals are to the inhabitants of a country what seas are to nations ; they equally serve toassist the wants of society and benefit commerce. The great navigations extend over thewhole globe, and transport the products of all nations and climates where they are in excessto where they are most needed, which, by means of the internal canals, are distributed tothe various districts in smaller craft, enabling these latter at the same time to maintaina continual and economical interchange of whatever may be required either for food ormanufacturing purposes. But the formation of a canal is frequently attended with greatdifficulties and all the talent of the engineer is called forth in overcoming obstacles ap»parently insurmountable, but which by a careful survey and patient investigation will gene-rally he vanquished.

In commencing a canal the first consideration should be directed to the quantity ofw ater that will be required, and from whence it is to be supplied, the means of keepingthe bottom clear, the construction of the sluices, and the distribution of the slopes of itsbed.

The water is usually collected from springs or rain, or derived from some neighbouringriver ; and in whatever way the supply may be received, it is incumbent on the engineercarefully to examine not only the nature of this supply, but also to ascertain the quantitylost by evaporation and absorption: should it be drawn from a river, it is commonlyeffected by constructing a weir across it, and after damming up the stream, admittinga portion into the canal, and, if a continuance of a navigation be not practicable in thebed of the river itself, the stream must be traversed by other dykes or weirs, the height°f which, as well as the depth of water required, will demand great consideration. Thesmallest possible quantity consistent with the purposes of navigation should be ad-mitted, in order to keep down the height of the sluices, banks, and dykes, all of whichare affected by the pressure of an increased body of water; and it is highly necessary tomake such arrangements that no injurious consequences shall arise from the floods, whichoccasionally swell the waters in the rivers of supply. Another important precaution1S preventing any deposit of gravel or other matters held in suspension by the water,whilst it is permitted to move rapidly forward: could it be freed from these before theirintroduction into the canal, much manual labour would be saved, and the fatal effects arisingirom the miasma produced by throwing out the mud on the banks be prevented.

The common method adopted in Italy for cleansing the canals is by sluicing: bottom orground sluices are constructed in the banks nearest to the river of supply, having their sillsaid at a considerable depth below the bottom of the canal, over which the water, beingoccasionally allowed to fall, acquires sufficient velocity for a considerable distance beyond

e opening of the sluice to detach any substances that may have lodged at the bottom of thewhei- T mGrOUS s * u * ces distributed throughout the whole length, placed at distances™ C cf,?iw ey ° ai *i ? e ma< * e to operate after the action of those below has ceased, have sue-CES " “° ved 'Vg? quantities of gravel deposit.

, w *i, 1S " rst ac . cumu lated in a large reservoir, the deposit may be made to

. a £ ,, ,! e water is used for the purposes of navigation: the expense attend-

ing this method has been its great objection: wherever gravel and coarse substances arebrought down by the river that feeds the canal, their diffusion should be as much as pos-sible prevented; this is sometimes done by opening an ample bottom gate immediatelyabove the sluice, and making the platform slope considerably towards the mouth of theoutlet; by keeping up the sill of the first gates, a little above the level of the outerplatform, when they were opened the gravel would be prevented from entering; but thismust be guided by circumstances, as no obstruction must be allowed to the navigationwhen the waters are low in the canal.

Level Cutting. — This term is applied to an excavation when the banks are made of theGarth taken out; it is the most economical method, and consequently the most generallypractised, the breadth and depth of the bank being of course dependent on the strength re-quired. It often occurs that a canal is cut on a hill, and a bank is made only on oneside; the work is then exceedingly simple, and called side or oblique cutting.

The Centre of Cutting is that point in a transverse section through which, if a line bedrawn representing the surface of the ground, the part for excavation will be found equalto that of embankment. To apply this to a canal on the side of a hill with parallel slopes,■^here one bank only is required, let A B C D be the section of the intended canal, CDEGFthat of the bank; to find the centre of cutting, continue the lines B C and D E to G and