(26.)

Whenever the air contained in any vessel or other enclosed space has by any means had its pressure reduced so as to be rendered less than that of the external air, the external air will have a tendency to rush into such vessel or enclosed space with a force proportionate to the excess of the pressure of such external air over that of the air within; and if any communication be opened between the interior of such vessel or enclosed space, and the external air, the latter will rush in until an equilibrium be established between the pressures within and without. It is evident that the force thus obtained by diminishing the pressure of air within a vessel may be applied to any mechanical purpose.

It is by such means that water is raised in an ordinary pump. A portion of the air contained between the piston of the pump and the surface of the water below, is withdrawn by the action of the piston, and the pressure of the air remaining under the piston is thereby diminished. The superior pressure of the atmosphere upon the external surface of the water in the well then forces up a column of water in the pump-barrel, and this is continued as the air is more and more rarefied by the action of the piston. By whatever means, therefore, the air can be wholly or partially withdrawn from any space, a mechanical power will be thereby developed, proportional in its amount and efficacy to the quantity of air so withdrawn. If, however, such air be withdrawn by any mechanical process, such as by a syringe, by a common pump, or by an air-pump, the quantity of force expended in withdrawing it is always equivalent to the amount of mechanical power obtained by the vacuum or partial vacuum so produced. Indeed the power expended is greater than the power so obtained, inasmuch as the friction, leakage, &c. of the exhausting apparatus must be allowed for.