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._ ══╤═══════════════════════╤═══════╤══════╤═════════╤══════╤════════ │ │ │ │THICKNESS│LENGTH│KIND OF │ │HEIGHT.│WIDTH.│ OF │ IN │MASONRY. │ │ │ │ ARCHING.│YARDS.│ ──┼───────────────────────┼───────┼──────┼─────────┼──────┼──────── │ │FT.IN. │FT.IN.│ FT.IN. │ │ 1 │Thames & Med. Canal │ 39.0 │35.6 │ ... │ 3960 │ BR’K 2 │Islington, Regents Can.│ 21.6 │20.6 │ 1.6 │ 900 │ “ 3 │Tetney, Haven Canal │ 16.2 │17.0 │ 1.2 │ 2962½│ “ 4 │Walford, N.W.R.R. │ 26.6 │27.0 │ 1.6 │ 1830 │ “ 5 │Box Tunnel, G.W. “ │ 36.0 │36.0 │ 2.3 │ 3121 │ “ 6 │Littleboro’, M.&L. “ │ 27.6 │27.0 │ 1.10½ │ 2860 │ “ 7 │Thames, Foot Passage │ 2.3 │37.6 │ 2.6 │ 400 │ “ 8 │Bletchingly, S.E.R.R. │ 30.0 │30.0 │ 1.10½ │ 1324 │ “ 9 │Saltwood, “ “ │ 30.6 │30.0 │ 2.3 │ 954 │ “ ──┼───────────────────────┼───────┴─┬────┴───┬─────┼──────┴──────── │ │ TOTAL │COST PER│YEAR │ MATERIAL │ │ COST. │ YARD. │WHEN │ CUT THROUGH. │ │ │ │BUILT│ ──┼───────────────────────┼─────────┼────────┼─────┼──────────────── │ │ DOLLARS.│ DOLLS. │ │ 1 │Thames & Med. Canal │ ... │ 145.00│1800 │Chalk,