1556

THEORY AND PRACTICE OF ENGINEERING.

Book IT.

Fig. 2897. section at CD.

Fig. 2896. section at AB.

in the middle of the shoulder walls for its issue: perhaps there would be a greateradvantage in making the tube in the side walls of the chamber, above and below, therecesses of issue being higher than under the wall of fall: there is no fear that the strikingof the water against the lower part of the wall of fall will in any way injure it.

In the pipes through which the water issues it rushes violently, but does not rise veryhigh, as its expenditure at the mouth is greater than the quantity which enters at the samemoment, and it may be rendered less rapid by making the tube larger at its exit than atits entrance. The recesses at the exit should be much larger than those of its entrance,and should be formed of large stones well cramped together, so as not to be easilyderanged; the platform should also be well cramped, as well as the stones of the archunder the wall, and the wall at the bottom of this recess should be tied in with that of theupper guard platform.

Bridges and Towing-paths .— The former may be made either above or below the lock-gates ; by placing them above, on the shoulder walls, the whole lock is free; and if theboats are loaded high, they meet with no impediment, but in this case, the balance beamsmust be dispensed with, but if the collars are well fitted, the gates will manoeuvresufficiently well; the shoulder walls must also be of an additional length, which increasesthe expense. By placing the bridges over the gates, and on the upper parts when opened,the expense is diminished; and as the load of the boats is seldom placed at the ends, whichare left free for working them, this cannot in any way affect their passage ; the onlyobjection is that the lock-keeper has to pass over the roadway to shut or open the lowergates, which is, however, avoided by making a small vaulted passage behind or under theabutments of the bridge.

The Towing-path may be made at the side of the lock in two ways, one by commencingthe ramp after the lower shoulder walls, in order to have a great platform at the side of thelock, and the other by making this ramp along the lock itself. The first method has thebest effect, but is objectionable, as the height of the wall of fall requires the talus to bevery steep, and covered with masonry. The second method is the most convenient;the banks of the canal being always at the same height, there is great economy in theexcavation as well as in the masonry ; the towing-path in both cases forms a right line, butthe platform of the lock is not so wide in the second as in the first.

Stop-gates and Lets-off. — Between Corpach and Loch Lochy, at Sirone, on theCaledonian Canal , is an outfall of three sluices, which prevents any injury occurring to thecanal banks from acci-dental breaches; they arein such a position at theend of the embankmentthat the water in thecanal can be dischargedwhen required for thepurpose of repair. Theopen fan or trough is 80feet at the widest end;that of the channel 18 feet,and the diameter of thesemicircular pool about82 feet: the three sluicesshown in the elevation areeach in width 4 feet, andin height 3 feet; theirsills are on a level withthe bottom of the canal,and the water falls 9 feetbefore reaching the bed

Fig.2898.

SLUICE AT SIBONB.