If the load upon the piston be increased to 30lbs, making with the pressure of the atmosphere 451bs, the pressure of the steam will be trebled, the piston will be raised 4 inches in the cylinder, and the effect will be 144 X 15 through the space of 4 inches, exceeding the former effect in the ratio of 120 to 90: had the load been increased to 451bs, exclusive of the atmosphere, the piston would have been raised through 3 inches, and the effects would have been as 135 to 90.

The cause of this increase of mechanical effect, by an increased pressure of steam, is, that in each Instance, in addition to the mechanical load to be moved, there is the pressure of the atmosphere to be overcome, which in the first case amounted to one half the whole load, but in the last case to only one fourth of the whole load; the greater the pressure therefore of the steam, the greater the mechanical effect, or rather the smaller the loss of effect. But considerations of safety, and other circumstances, prevent this principle from being carried to a great extent in practice.

To illustrate the principle upon which the condensation of steam tends to produce a mechanical power; let us suppose that in the before described cylinder, the cubic inch of water has formed a cubic foot of steam, the pressure of which just balances the pressure of the atmosphere upon the piston, which will then be raised one foot high in the cylinder; if by any means the steam be suddenly condensed and reduced to one cubic inch of water, a vacuum will be formed beneath the piston, and the pressure of the atmosphere upon its surface, being no longer balanced by that of the steam beneath, will carry the piston to the bottom of the cylinder, with a force of 151bs upon each square inch of its surface, or 2,160lbs in the whole. And if a weight of 2,160lbs be supposed to be attached to the line to which the counterbalance is hung, it will, by the descent of the piston in the cylinder, be raised one foot high; the mechanical effect therefore in this case is 2,160lbs raised 1 foot high. The above may be taken as an illustration of the principle of the atmospheric engine, as it is termed, which is the first form of the steam engine, in which the force of the steam was transmitted through a piston.

As respects the third mentioned cause of motion in steam engines, viz. the force produced by its pressure, combined with thatresulting from its condensation; - the pressure of the steam is substituted for the pressure of the atmosphere, to impel the piston into the vacuum formed within the cylinder by the condensation of the steam. In this case the reader must suppose the cylinder before described to be fitted with a steam-tight cover, through a hole in which a rod attached to the piston works steam-tight, and that upon this rod is placed the load to be raised. If a vacuum be formed above the piston, and the cubic inch of water below the piston be converted into steam, the piston will be raised through a space which will be inversely as the amount of the load; thus, if the load be equal to 151bs upon the square inch, it will be raised 1 foot high; if 30lbs per square inch, it will be raised only 6 inches; and so on, the product of the weight, multiplied by the space through which it is raised, being equal in all cases. Theoretically considered,.therefore, the mechanical effect of a given quantity of water, converted into steam, is the same whatever the pressure of the steam may be; and is equal to that produced by the effect of the pressure of the atmosphere against a vacuum.

But it is advantageous on many accounts, in practice, both to substitute the pressure of the steam for that of the atmosphere to impel the piston, and also to increase its elastic force beyond that of the atmosphere.

With respect to the fourth cause of motion in engines; - if steam be made to flow from a centre along a hollow arm, and to issue in a jet, near the extremity, in a direction at right angles to such arm, it will impart a rotary motion to the arm in an opposite direction to that of the jet. This motion is attributed by many to the "re-action" of the steam against the atmosphere, which is supposed to form the abutment to the steam. But this explanation is clearly incorrect, since engines constructed upon this principle of action will revolve in a close casing in which a vacuum it maintained.

Another, and, as it appears to us, the true theory, is that it results from a difference of pressure on the opposite side of the arms; which may be illustrated thus. Let a a in the diagram be supposed to represent two hollow arms, capable of motion upon a hollow axis 6, through which steam is admitted to the arms. If these arms have no outlet, the pressure of the steam will be the same in every part of their surface; and there being no issue of steam, no motion will take place. But if a small aperture be made in each arm at c c, the steam will rush out through them, and the pressure upon that side of the arms will be removed; whilst it will continue to be exerted upon the opposite points of the arms at d d, and the arms will therefore revolve in the direction of the arrows. Having now explained the theory of the several causes of motion in the steam engine, we shall proceed to show the practical application of them, by the description of a series of different engines, that are actuated by those several causes of motion; in doing which we shall introduce them in the order of their discovery, our space not admitting of a historical detail of the successive steps by which the steam engine became the powerful and elaborate machine which it now exhibits.

The earliest contrivance of which we have any account, for making use of the force of steam, is of unknown date. It is generally called Hero's engine, and though posterity is really not indebted to him for the invention, it is still more beholden to him for the bequest of his description, than if he had been the inventor and omitted to describe it.