564

HISTORY OF ENGINEERING.

Book I.

In this situation it was not deemed practicable to form a wooden cofferdam sufficientlylarge to construct a lock 170 feet in length, and 40 feet in width, besides the necessary wing-walls, and the following method was therefore adopted. An iron railway was laid down,on which the heavy mountain clay found close by was carted, and the two banks of thecanal were first formed as far as where the depth of water at an ordinary neap tide was20 feet; and when the site of the intended lock was approached, the banks were united intoone mass ; the mountain clay compressed the soft mud beneath and squeezed out the water.Upon this great mound of earth a quantity of stone was laid, which was afterwards used inthe construction, and remained for six months, at which period the mound had sunk 11 feet,and it being considered that no further material sinking would take place, a lock pit wasexcavated in the solid mound, and the water was kept down by a steam-engine of ninehorse power. When the lock pit was excavated, rubble stone masonry was laid withhydraulic mortar to the thickness of 2 feet in the middle of the lock chamber, increasing to5 feet thick on each side ; upon this an inverted arch of square masonry was struck, and onit was built the side walls ; the chamber walls, counterforts, recesses, and wing-walls werethen carried up.

The masonry of the bottom part was built in lengths of 18 feet, in order that the mudmight be prevented from again rising up in the space newly compressed. The mud waseasily penetrated by piles, but after they had been driven a few hours, no power could eitherdrive them further or draw them out. The whole of the masonry was completed in 1812,the rise of the lock being 6 feet 8 inches. The system adopted was found to have beenfar less expensive than a cofferdam.

The five connected locks at Fort Augustus have their foundation on a coarse open gravel,and to construct them it was found necessary to turn the river Oich. A trial pit wasthen sunk by means of a steam-engine of six-horse power ; but when a depth of 18 feet wasobtained, the water accumulated so fast, that a pump well and an engine of 36 horse-powerwere required, which enabled the lock bottom wings and forebay walls to be constructed,though during the progress of these works, a third engine of nine horse-power was used,and when the depth arrived at was 25 feet below the surface of Loch Ness , the whole threeengines were required to keep the waters down.

There is a let-off or outfall between Corpach and Loch Lochy, at Strone, which consistsof three sluices, each 4 feet broad and 3 feet high, the sills being on a level with the bottomof the canal, and the inside faces of the water surface ranging with the bank line; the waterfalls 9 feet before it reaches the rock on which it falls ; the frames and sluice doors are ofcast-iron, the working parts being faced with copper ; the sluice doors are raised and loweredby rack-work, enclosed in cast-iron cylinders, placed over the centre of each, and rising1 foot above the surface of the canal when full.

The length of the canal operated upon by this let-off is 6 miles, where it is 80 feet wideat the surface, 50 feet at the bottom, and 10 feet deep, and the three sluices when openedlower the water as follows : —

From 10 feet to the depth of 9 feet

9 - - - 8

8 - - - 7

7 - - - 6

6 - 5

5 - - - 4

hr. min.1 39

1 42

1 45

1 49

2 6

2 38

The freshwater lakes form 37^ miles of its length, and vessels are admitted drawing

15 feet water.

The total expenditure from its commencement in 1803 to May, 1829, was— £

For work performed by contract and measurement ----- 652,494Ditto, by day work --------- - 68,099

Expenses of management for timber, machinery, shipping, land purchased, &c. 261,766

£982,359

The price of Baltic timber at its commencement was 2s. 6d ., and got up to 7s. per footcube. The lock gates are of English oak and cast-iron ; there are fourteen locks on thewest side, and thirteen on the east side of the summit. The turn-bridges are all cast-iron.

Carlisle Canal-, 11£ miles in length, was executed under the direction of William Chapman,a native of Whitby, at a cost of about 120,000/. It commences on the south-eastern sideof Carlisle, and falls into the sea through a height of 70 feet, by means of 9 locks. AtCarlisle there is a basin of considerable extent, and the first reach is 4 miles in length : itthen falls 46 feet by 6 locks, in the length of the next miles : it is then nearly level toBowness, and afterwards falls into the sea by three locks, the lift of each being 8 feet;between these are two basins, called the Upper and Lower Solway ; the latter is on a level