To insist that the canal must possess the character of a strait, may give rise to the necessity for a thorough-cut of such extreme depth, or a tunnel of so great length, as to render the work practically impossible. A line suitable for a thorough-cut may possibly be found, but so important a project should not be endangered by limiting its practicability to a communication of that nature. If, by the employment of “lift-locks,” the cost of the canal can be materially reduced, the question to be considered is, to what extent such structures would obstruct navigation? This question depends upon the amount of trade drawn to the Isthmus by the canal. The relative cost of the two methods for piercing the Isthmus can be best determined by a comparison of the cost of a canal in an open country with one by means of tunnels. These considerations, since they afford criteria for judging of the merits of different routes, may be considered more minutely. Let us assume the trade passing over the Isthmus—were the canal now completed—to increase one hundred per cent. in ten years; there would then be 2,066 tons in transitu daily, requiring seven ships of about 300 tons burthen each.[5] The progressive increase in the size of ships will raise this average to between 500 to 1,000 tons; reducing the number of ships arriving at the Isthmus daily, to five and three respectively. But, assuming the smaller average, giving the larger number of seven ships daily passing through the canal; an increase of four hundred per cent. in the trade would be equivalent to fourteen ships, or to seven ships leaving opposite extremities of the canal, and passing each other daily upon homeward and outward voyages. Locks of four hundred feet long by ninety feet wide can be filled or emptied in twenty minutes; and this time can be reduced for smaller vessels by additional lock-gates, and for larger vessels by an increase in the size and number of filling valves. The entire trade likely to seek this route, increased four hundred per cent. of its present amount, could be passed through one lock in about four hours and forty minutes. As the vessels come from opposite directions, one-half of the number would be waiting for lockage at the same point, which would reduce the time required for this purpose to two hours and twenty minutes. Eight locks, having an average lift of twelve and one-half feet, would delay the increased commerce eighteen hours and forty minutes, and would raise the level of the canal fifty feet; while to raise the level one hundred feet the delay would not exceed two days.[6] As a summit level may be a necessary part of any Isthmean canal, it is manifest that the resulting lockage can not seriously obstruct navigation. The design of an artificial strait may therefore be reasonably abandoned, if, by so doing, the extraordinary cost of tunneling is excluded by the employment of a small number of lift-locks.