In our last article on this subject we pointed out the reasons why the gas engines which had been invented to supersede steam had all failed to compete with it. We will now proceed to give the reasons why Hot Air has hitherto failed and must continue to fail in competing with steam as a force to move machinery. All bodies in nature exist in either of three conditions, viz., solids, liquids, and gases. Different laws govern these three different conditions of matter, and many bodies can be easily converted into any of these three conditions. A certain amount of heat applied to ice will change it into a liquid, and the application of a greater amount of heat will change it into vaporsteam. All bodies suffer a temporary increase of dimensions when heated, and contract again into their original volume on cooling. It is this feature of the expansion of bodies by heat, which enables man to employ them to move machinery. Thus by heating bars of iron the walls of a building in Paris were brought straightthrust from ' an incline to a perpendicular ;thus by hea ing air in a balloon, Montgolfier was enabled to force his way upwards against the pressure of the atmosphere, and by heating water till it became steam, Hero was enabled to whirl round his primitive rotary engine. Now the question which we are to discuss, is what body in nature is the most economical as a motive force, whether a solid, liquid, or gas. There is no use in losing time speaking of the solid, therefore we will speak only of liquids and gases, and only of one liquid and one gas water and air. The principle of any chemical force to propel machinery depends on the nature of the substance employed ; thus water by heat being applied to it, expands to 1,728 times its bulk ; it is this elastic torcewater combined with heatwhich moves the piston in the cylinder of an engine. Air by having heat applied to it also expands, and this expansive force admitted under the piston of an engine, will also move it. Now, if water and air expanded equally with the same amount of heat applied, and were otherwise alike easily condensed to their original bulk no one could doubt the propriety and economy of using air in place of steam as a motive power, but this is not the case. Steam and air alike, come under the law of Mariotte in expanding their bulk equally with the same amount of heat applied, namely, doubling their volume for every 491 of heat applied. A cubic foot of air at 32 cannot move the piston of a cylinder, but if 491 of heat are applied it will occupy double the space and lift 2,160 lbs. one foot high at the pressure of the amosphere, that is exerting a pressure of 15 lbs. on every square inch of a piston of 144 inches area. But a cubic foot of water at 212 sensible heat converted into steam at the same pressure will lift 3,732,480 lbs. one foot high. Air is not for a moment then to be compared to water, bulk for bulk, to exert elastic force by the application of heat ; this is evident, for the latent and specific heat of steam is only 1,184. Allowing it to be safe to employ heated air (but it would not) about 491 to double its original volume, it will require 864 cubic inches of air, at 32, to which 491 have been applied, to equal one cubic inch of water raised into steam from 32. Thus there is a vast diffe-tence between a liquid and a gas to which heat have been applied, in the expansion of their bulkstheir elastic force. To make air triple its volume, it would require to be heated to 982a low red heat. In its very nature, steam seems to have been designed by a Wise Providence, as a mighty power suited for the propelling of machinery, for while it contains 1181 of heat (latent and specific combined) yet it has only 212 of sensible heat, and in this respect combines a heat (and consequently a force) of an intensity and in such a form as dare not be applied to air. The boilers of the steamship Atlantic evaporate 7J lbs. of water by one of coal, that is 207'36 cubic inches of water, which by such a small amount of coal is converted into 358,-318'08 cubic inches ofsteam. Will one pound of coal thus expand 207 cubic inches of air to 1,728 times its original bulk at 32 ? Unless it does it is more expensive than steam, and besides steam can be condensed at an expence of only three fifteenths of the power of atmospheric resistance, and air cannot be condensed at all to its original volume, until the whole of its heat is abstracted. Thus from its very nature water has many advantages over heated airthe fluid over the gas. No wonder all the Hot Air Engines hitherto invented have failed to compete with it. We have not said anything yet about the exhaustion of expanded gases from engines, the principle and cause by which they are enabled to act ; we will| do so next week. It would extend this article to an undue length, to add what we have to say now, every article, however, is complete in itself.