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CHAP. XII.



THE MODE OF CROSSING RIVERS AND GAINING HEIGHT AT THE

 SAME TIME, PERFORMING THE DOUBLE OPERATION OF AN 

AQUEDUCT AND LOCK. (PLATE VII.)





	This apparatus is the double inclined plane in all its parts, either for a descending or alternate trade; the stone piers supporting iron rails which compose the plane.



	It would be difficult to draw the exact the exact difference, in expense, between this machine and an aqueduct to cross a river, than locks to gain height; but if the immense labor in constructing an extensive and high aqueduct is considered, I conceive the saving will be found very important. The great quantities (which should be of a good quality), hewn to certain squares and templates; the frequent long carriage, the timber for centers, and the various preparatory works, all tend to load aqueducts with heavy expenses, and render them one of the greatest obstacles in prosecuting canal.



	If I suppose it necessary to carry a canal one hundred feet high, and three hundred feet in length, over a river, it is a moderate calculation, considering the average of situations, to estimate it at the round sum of										L 20,000



To which add one hundred feet lockage for twenty-five ton boats,			     7,000

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Amounting to									   27,000



Yet a similar situation may be crossed, and the height gained, in one movement, as appears by calculation, for about 8,000l, ; consequently 19,000l is saved, which is almost the whole expense of the aqueduct.



	Without entering into tedious estimated to prove this, I conceive the common sense of those who have the least knowledge of the subject, will inform them of the reason of these savings; but to the speculating reader, if he observes by the plate that the part crossing the river is a simple stage, the weight of which, compared to the load of water and stone which press on the piers of an aqueduct, is trifling; consequently the piers are but as columns without arches, and the masonry, which in aqueducts is almost the whole expense, is here so contracted to be, comparatively, scarce worth mentioning; iron arches or braces being adopted, the expense of which is not more than the centers to build an arch of stone, and the continued plane performing the part of locks, of which he has had a comparative view, I conceive it will exhibit the obvious reason of such important savings.



	This compilation is indeed so material, that in projecting a line of canal where a river or deep valley is to be crossed, it should be well considered, whether it can conveniently be brought to such a situation that the ground will admit of rising at the time of crossing; which, even on the small system, will save time and expense.