As a corollary to this law, it follows that many a force of nature, hitherto neglected because of its position or intractability, may be turned to practical account by using it to produce some new power, which may be either stored up or transmitted to a distance, and so can be employed wherever and whenever it is required. Thus, in the first machine we propose to notice, a M. Cazal has just hit upon a plan by which to use the power of falling water at a considerable distance. He employs a water-wheel to turn a magneto-electric machine (of the kind used for medical purposes, on a very large scale), and the electric force so obtained may be conveyed to any distance, and employed there as a motive power. In this way a mountain stream in the Alps or Pyrenees may turn a lathe, or set a loom in motion, in a workshop in Paris or Lyons; or even (as has been remarked), if a wire were laid across the Atlantic, the whole force of Niagara would be at our disposal. The idea is at present quite in its infancy; but we are told that the few experiments hitherto made show that such an engine is not only very ingenious but perfectly feasible, and (most important of all) economical. The second engine gave promise of considerable success when first brought out in Paris about eight months ago. It was invented by a M. Tellier, and proceeds on the principle of storing up force, to be used when wanted. It has long been well known to chemists that a certain number of gases (as chlorine, carbonic acid, ammonia, and sulphuretted hydrogen) can be condensed into liquids by cold or pressure, or both combined. Of all these gases, ammonia is the most easily liquefied, requiring for this purpose, at ordinary temperatures, a pressure only six and a half times greater than that of the atmosphere. A supply of liquid ammonia obtained in this manner is kept by M. Tellier in a closed vessel, and surrounded with a freezing mixture, so that it has but little tendency to return to the gaseous state. A small quantity is allowed to escape from this reservoir under the piston of the engine, and, the temperature there being higher than in the reservoir, the ammonia becomes at once converted into gas, increasing thereby to more than twelve hundred times its previous bulk, and so driving the piston with great force to the top of the cylinder. A little water is now admitted, which entirely dissolves the ammonia, a vacuum being thus created, and the piston driven down again by the pressure of the air without. M. Tellier employs three such cylinders, which work in succession; and the only apparent limit to the power to be obtained from this machine is the amount of liquid ammonia which would have to be used, about three gallons (or twenty-two pounds) being required for each horse-power per hour. There is no waste of material; for the water which has dissolved {31} the gas is saved, and the ammonia recovered from it by evaporation, and afterwards condensed into a liquid. M.