The watertight compartments at the lower part of the caisson, as well as the bottom portion, arranged to serve as a working-chamber, were to communicate with the outer air by shafts, provided with air-locks, so that water or compressed air could be introduced at pleasure. This arrangement would enable the counterpoise of the caisson to be readily adjusted, the different chambers to be easily reached for repairs, and the working-chamber at the base to be used for cleaning the sill from silt or _débris_. The caisson gates, in a lock of 36-1/11 feet lift, would be 69 feet high, 71 feet long, 13⅛ feet broad at the tail, and 32¾ feet high, 71 feet long, and 9⅚ feet broad at the head of the lock. The locks, being situated in rock, would have the sides of their chambers formed of the natural rock, with a slight facing of masonry where necessary; but the side walls below the gates were to be iron caissons, 18 feet broad, filled with concrete. The swing-bridges, of iron or steel, were to be 18 feet wide, and 112 feet long, the swing portion being 78 feet; and the recesses for the caissons were to be 98½ feet long; and 23 feet wide at the top. The filling and emptying of the locks were to be effected by two cast-iron pipes, each 9⅙ feet diameter, and it was calculated that the required volume of 52,300 cubic yards of water could be admitted or shut out in fifteen minutes.[188] _Special Features of the Enterprise._—Probably no great engineering or constructive work of either ancient or modern times has been of such a gigantic and difficult character as that of the canalisation of the Isthmus of Panama. It is not that the length of the canal is exceptional; it is, on the contrary, shorter than that of many existing canals, some of which are of very small account indeed—being less than one-half the Suez Canal, less than one-third that of the canal of Languedoc, and less than one-fourteenth that of the Grand Canal of China. It is probable, also, that the building of the Great Wall of China, the Pyramids of Egypt, and several other works of antiquity that might be named, extended over a much longer period, and involved the employment of a greater number of men. The _Royal_ or _Grand Canal_ of China, which was completed in the year 980, is said to have occupied the labour of thirty thousand men for forty-three years.[189] But none of the great works of previous epochs have been environed with so many difficulties as the Panama Canal. The scheme, on the face of it, does not look so formidable. It is only when we come to look into its details, and compare them with those of other similar undertakings, that we realise its magnitude. And it is only when we, in like manner, compare its engineering features with those of the other great engineering works of the world that we can appreciate the vast energy, enterprise, and resource that has ventured to essay so colossal a task.