Therefore, supposing one of the tanks is at the highest point of one of the shafts, the other will naturally be at the lowest level of the other shaft. The upper tank is supported by compressed air resting on a body of water which fills the lower portion of the shaft--that is, the part sunken down into the earth. All that it is necessary to do now in order to bring the upper tank or elevator down to the level of the lower tank is to open a valve, and allow the compressed air to run out of one shaft into the other, which it will do at a velocity twenty-eight times that of water. The weight of the descending tank, of course, is the power which forces the air through the pipe from one shaft into the other, and as soon as the two tanks reach the balancing-point they will stop. In order to get the elevator down to the bottom level, therefore, it is necessary to allow water to run into that compartment which needs to be made the heavier and to allow water to run out of the other. It is plain to be seen, therefore, that this invention is bound to crowd out the old-fashioned stationary canal lock if it can be constructed cheaply enough. [Illustration: PNEUMATIC LOCKS IN OPERATION.] The canal lock of to-day is a very slow working affair, as we all know, and is such a clumsy piece of mechanism that only ships of a limited tonnage can pass through it. When a canal-boat comes along it is let into the first lock, and if that is on the higher level the gates are closed, and the water is allowed to run out until the boat is floating on a level equal to that in the other portion of the lock. Then the gates are opened, and the boat passes on. If it is necessary, on the other hand, to raise the vessel from the lower to the higher level, much more time has to be consumed in order to pump the lock full of water. By Mr. Dutton's method, however, the vessel comes along the canal, and it may be as large a ship as an ocean freighter, and it may carry as great a cargo as 12,000 tons, and yet it can slip into one of the great steel tanks 510 feet long, and a boy can open the compressed-air valve, and let the great ship travel gently down the elevator shaft until it reaches the lower level of the canal. The operation requires perhaps fifteen minutes, instead of hours; and no more time is necessary for ships of equal tonnage going in the other direction, since a much greater weight of water can be run into the upper tank from the higher level of the canal than could be counterbalanced by any kind of steamship that would need to be lifted from the lower level. A company has been organized to build a canal from the Atlantic to the Great Lakes, and it is its intention to use Mr. Dutton's locks along the way; not more than two or three will be necessary. But as it will cost about one hundred million dollars to carry out the enterprise, it may be some years before they will be able actually to begin work.