of power to any considerable distance. It has been found, for instance, in attempting to blow a furnace by means of a powerful water-wheel driving air through a cast-iron pipe of above a mile in length, that scarcely any sensible effect was produced at the opposite extremity. In one instance, some accidental obstruction being suspected, a cat put in at one end found its way out without injury at the other, thus proving that the phenomenon did not depend on interruption within the pipe.
(351.) The most portable form in which power can be condensed is, perhaps, by the liquefaction of the gases. It is known that, under considerable pressure, several of these become liquid at ordinary temperatures; carbonic acid, for example, is reduced to a liquid state by a pressure of sixty atmospheres. One of the advantages attending the use of these fluids, would be that the pressure exerted by them would remain constant until the last drop of liquid had assumed the form of gas. If either of the elements of common air should be found to be capable of reduction to a liquid state before it unites into a corrosive fluid with the other ingredient, then we shall possess a ready means of conveying power in any quantity and to any distance. Hydrogen probably will require the strongest compressing force to render it liquid, and may, therefore, possibly be applied where still greater condensation of power is wanted. In all these cases the condensed gases may be looked upon as springs of enormous force, which have been wound up by the exertion of power, and which will deliver the whole of it back again when required. These springs of nature differ in
