There is another error in Mr. Gisborne’s statement. “The tide,” he remarks, “would take one and one-half hours to reach from one end to the other, presuming the current to be uniform; what,” he asks, “will be the surface velocity in a canal thirty miles long?”

This statement contradicts his calculations, and involves also the question at issue. If the tide travels to the end of a canal thirty miles long in “one and one-half hours,” it is evident that it must move at the rate of twenty miles per hour, a velocity which renders Mr. Gisborne’s strait impracticable for navigation.

In fact, neither assumption is tenable. The problem is very complex, or, rather, with the data given, indeterminate. It is well known that the tide is propagated up the channel of a river in a succession of long waves, or swells, and that when the tidal wave is entering the mouth of the river, the waves which have reached the head are returning. The same movement is observed, on an exaggerated scale, in the successive breakers which roll in to meet the one which is returning, after it has expended its force upon the beach.

In the case of the Isthmean Canal, the rising tide, after having passed the mean, will have a downward slope into the canal. In rivers, notwithstanding the local rise of the water, the slope is never reversed, but is simply reduced in its angle of inclination.

The problem involves the inclination of the surface, or the determination of the limits of tidal action at successive stages of the tide. While the head of water increases, there is also a constant increase of the retardation of the flow of water into the canal. The outflowing water will run more rapidly than the inflowing, on account of the indefinite area over which it will spread and the diminution of the retarding influences. Both outflowing and inflowing current will seriously obstruct navigation. The banks of the canal will wash away, and bars will accumulate about the mouth.

While these objections are valid against a thorough-cut canal without locks, they do not apply to a strait of a quarter of a mile in width. As the cost of a canal is the chief difficulty in the way of its construction, it is necessary to abandon the idea of a strait, and to adopt that of a thorough-cut with guard-locks, as the only known means of protecting the canal from the injurious effects of the tide.

In order to form a correct opinion of the cost of canals with and without tunnels, attention is called to the expense incurred in the execution of this kind of work.

Dimensions and Cost of some English Tunnels.

HEIGHT. WIDTH. THICKNESS
OF
ARCHING.		 LENGTH
IN
YARDS.		KIND
OF
MASONRY.		 TOTAL COST. COST PER
YARD.		YEAR
WHEN
BUILT. 		MATERIAL CUT
THROUGH.
FT.IN.FT.IN.FT.IN. DOLLARS.DOLLS.
1 Thames & Med. Canal39.035.6...3960 BR’K...145.001800Chalk, Fuller’s earth.
2 Islington, Regents Can. 21.620.61.6900“......1812London clay.
3 Tetney, Haven Canal16.217.01.2  2962½ “ 563,405192.501827Various.
4 Walford,  N.W.R.R26.627.01.61830“......1838Chalk.
5 Box Tunnel, G.W. “36.036.02.33121“1,561,500500.001838Freestone.
6 Littleboro’, M.& L. “27.627.0  1.10½ 2860“4,255,000440.001841Various.
7 Thames, Foot Passage 2.337.62.6 400“2,273,5705,685.00 1842London clay.
8 Bletchingly, S.E.R.R.30.030.0  1.10½ 1324“ 486,185351.001842Shale.
9 Saltwood,   “ “30.630.02.3 954“ 562,710590.001843Lower greensand.

Canal tunnels are rarely larger than 16½ feet by 18 feet high. Supposing the same dimensions to obtain in French tunnels, the cost per lineal yard of the following named tunnels will furnish a basis for comparison: