if air or any other gas be augmented in temperature, it will likewise be increased in elasticity; and if, on the other hand, it be diminished in temperature, it will be likewise diminished in its elastic force. The more heated, therefore, any air or gas confined in a vessel becomes, the greater will be the force with which it will press on the inner surface of that vessel, and tend to burst it. Ex (14.) The same body may, by the agency of heat, be made to pass successively through the different states of solid, liquid, and gas, or vapour. The most familiar and obvious example of these successive transitions is presented by water. posed to a certain temperature, water can only exist as a solid; as the temperature is increased, the ice, or solid water, is liquefied; and by the continued application of heat, this water again undergoes a change, and assumes the form, and acquires the mechanical qualities, of air or gas: in such a state it is called STEAM. This is a common property of all liquids. If they be exposed for a sufficient length of time to a sufficient degree of heat, they will always be converted into elastic fluids. These are usually distinguished from air and other permament gases, which never are known to exist in the liquid form, by the term vapour, by which, therefore, must be understood an elastic fluid which at common temperatures exists in the liquid or solid state; by steam is expressed the vapour of water; and by gases, those elastic fluids which like air are never known at least, under ordinary circumstances to exist in any other but the elastic form. (15.) When a liquid is caused, by the application of heat, to take the form of an elastic fluid, or is evaporated, besides acquiring the property of elasticity, it always undergoes a considerable change of bulk. The amount of this change is different with different liquids, and even with the same liquid it varies with the circumstances under which the change is produced. (16.) When water is evaporated under ordinary circumstances, that is, when exposed to no other external pressure than that of the atmosphere, it increases its volume about seventeen-hundred-fold. Thus a cubic inch of liquid water would form about seventeen hundred cubic inches of common steam. If, however, the water be confined by a greater pressure than that produced by the common atmosphere, then the increase of volume which takes place in its evaporation would be less in proportion. These important physical circumstances are now only indicated in a general way. As we proceed with our account of the invention and improvement of the steam engine, they will be developed more fully and accurately. (17.) After duly considering what has been just explained, no difficulty will be found in comprehending the principles on which the first rude attempts at the mechanical application of steam already stated depend. In the apparatus ascribed to Hero of Alexandria, the elasticity of the vapour contained in the arms of the revolving ball causes that vapour to issue from the lateral orifices in the arms, such as that of G, fig. 1. As these orifices, however, are exposed to the common atmosphere pressing inwards with a force, the mean amount of which has been stated to be about fifteen pounds per square inch, it follows that the steam cannot escape from these orifices until its pressure or elasticity exceeds this amount, and that when it does, the force with which it will so escape will be the excess of its elasticity above that of the atmosphere; and it is the reaction produced by this difference of pressure, causing the arms to recoil, which will give motion to the machine. In the case of the apparatus of De Caus (5.), the heat of the fire acting on the vessel DC (fig. 2.) will raise the temperature of the water contained in it, and also of the air confined within it above the surface of that water. This air, as it is increased in temperature, will also increase in elasticity; it will therefore press on the surface of the water with increased force, and will gradually force the water upwards in the tube; and this effect would continue until all the water in the vessel would be forced up the tube. But at the same time that the heat acting on the vessel increases the temperature of the air above the water, it also produces a partial evaporation of the water, so that more or less steam is mixed with the air in the vessel above the sur face of the water; and this steam possessing elasticity, unites with the air in pressing on the surface of the water, and in raising it in the tube. Let us now revert to the brief account of the engine of the Marquis of Worcester, described in "The Century of Inventions." We collect from that description that the vessel in which the water was evaporated was separate from those which contained the water to be elevated; also that there were two vessels of the like description, the contents of which were alternately elevated by the pressure of the "water rarefied by the fire;" in other words by steam; and that the water was raised in an uninterrupted stream, by the management of two cocks communicating with these vessels and with the boiler. The following is such an apparatus as would answer this description. Let E (fig. 4.) F Fig. 4. R Fig. 5. a pipe to allow the steam produced from the boiling water in E to pass into the vessels where its mechanical action is required. Let R represent a cock or regulator, having in it a curved passage, leading from s to the tube T, when the lever or handle L is in the position represented by the cut; but leading to the tube T', when the lever L is turned one quarter of a revolution to the right, as represented in fig. 5. By the shifting of this lever, therefore, the steam pipe s may be made to communicate alternately with the tubes T and T'. The tubes T and T' are carried respectively to two vessels v and v', which are filled with the water required to be raised. In these vessels tubes enter at c and c', descending nearly to the bottom: these tubes have valves at в and в', opening upwards, by which water will be allowed to pass into the vertical tube F, but which will not allow it to return downwards, the valves B and B' being then closed by the weight of the water above them. Let G G' be a pipe entering the sides of the vessels v and v', for the purpose of filling them with the water to be raised: let κ be a cock having a curved passage similar to the cock R, and leading to a tube by which water is supplied from the reservoir or other source from which the water to be raised is drawn. When the cock K is placed as represented in fig. 4., the water from the reservoir will flow through the curved passage in the cock K into the tube G', and thence into the vessel v'; but when this cock is turned one quarter round, by shifting the lever to the left, it will take the position represented in fig. 6., and the water will flow through the curved passage into the tube G, and thence into the vessel v. Let us now suppose the vessel v already filled with water to be elevated, and the vessel v' to have discharged its contents. The cock R is turned, so as to allow the steam generated in the boiler E to pass into the tube T, and thence into the upper part of the vessel v, while the cock K is turned so as to allow the water from the reservoir to pass into the tube G', and thence into the vessel v'. The steam collecting in the upper part of the vessel v' presses with its elastic force on the surface of the water therein, and forces the water upwards in the tube c; it passes through the valve B, which it opens by the upward pressure received from the action of the steam, and thence into the tube F, its descent into the tube c' being prevented by the valve v', which can only be opened upwards. As the steam is gradually supplied from the boiler E, the water in the vessel v is forced up the tube c, through the valve B, and into the tube F, until all the contents of the vessel v above the lower end of the tube c have been raised. In the meanwhile, the vessel v' has been filled with water, through the cock K: when this has been accomplished, the man who attends the machine shifts the cocks R and к, so as to give them the position represented in fig. 5. and fig. 6. In this position, the steam from the boiler, being excluded from the tube T, will be conducted to the tube T', and thence to the vessel v', B', and into the tube F; its descent into the tube c being prevented by the valve B, which will then be closed. After the contents of the vessel v have thus been raised, and the vessel v replenished, the two cocks R and K are once more shifted, and the contents of v raised while v' is replenished, and so on. If, having comprehended the apparatus here described, the reader refers to the description of the Marquis of Worcester's machine, he will find that all the conditions therein laid down are fulfilled by it. One vessel (E) of " water rarefied by fire' may by such means "drive up forty (or more) of cold water; and the man that tends the work has but to turn two cocks, that one vessel (v) of water being consumed, another (v') begins to force and refill with cold water, and so on successively, the fire being tended and kept constant; which the self-same person may likewise abundantly perform, in the interim between the necessity of turning the said cocks." On comparing this with the contrivance previously suggested by De Caus, it will be observed, that even if De Caus D |