through the cylinder and piston. As some of our readers may never have heard of this contrivance, which has now become an essential part of the steam-engine, we will endeavour to give a short explanation of it.

The plan of moving weights by means of a cylinder and piston was invented long before the time of Papin. The contrivance, is a very simple one. It consists of a hollow tube or pipe, called a cylinder. This cylinder is fitted with a kind of plug, called a piston, so carefully made as to move easily up and down in the cylinder, without, however, permitting the passage of air, water, or any substance whatever, between itself and the cylinder. A rod is attached to the centre of the piston, by means of which the piston can be worked up and down in the cylinder. The other end of this rod is joined to a cross-beam, and the weight to be lifted, or the handle to be worked, is attached to the opposite end of the cross-beam. When the piston is at the bottom of the cylinder, the weight at the other end of the cross-beam instantly draws it up again. If, therefore, the piston could be forced down in the cylinder as often as the weight dragged it up again, a regular up and down movement, like that of a see-saw, would be given to the crossbeam, which would consequently draw up and down with it the pump-handle, or whatever else might be fastened to it at the end opposite the piston. This regular descending motion of the

piston in the cylinder was obtained by making use of one of the most simple and beautiful principles of nature.

The air or atmosphere in which we live, and which we breathe, is an invisible gas. Now, though we can neither see nor feel air, it is a very different thing to mere empty space, and it is possessed of one very important quality, namely, weight. Any substance which is lighter than air, will float on the air just as it does upon water; for the air supports or holds it up. If, however, the air could be got rid of, the substance would instantly fall to the ground. Of course, out of doors it is impossible to get rid of the air, but in a closed vessel the air can be exhausted, or, as it is termed, a vacuum (the Latin word for an empty place) can be created by various means. A celebrated German philosopher, named Otto de Guericke, who lived in the early part of the seventeenth century, and who made many important discoveries on the subject of air, invented an air pump, which was a contrivance for pumping air out of a vessel, and thereby obtaining a vacuum. An experiment was also made by the same ingenious person, which is very remarkable, as proving the extraordinary force or weight of air. He made a large copper globe composed of two hemispheres fitting closely together. By first filling this globe with water, he succeeded in expelling the air from it, when, although it was merely held together by

the pressure of the external atmosphere, six horses were unable to pull it asunder.

Now with regard to the cylinder and piston. If a vacuum were created in that part of the cylinder which is below the piston, it is clear that the piston, no longer meeting with any resistance, would, by its own weight and the pressure of the atmosphere above it, descend to the bottom of the cylinder. Such, then, was the principle by which the up and down motion of the piston in the cylinder was obtained. A vacuum was created in the cylinder below the piston by means of an airpump, the piston immediately descended in the cylinder, and, as soon as it reached the bottom, was instantly drawn up again by the weight at the other end of the cross-beam.

The method, however, of exhausting the air from the cylinder by an air-pump, was not at all effectual, besides being very laborious. It appeared to Papin that some other better means of creating a vacuum might be devised, and he employed steam for that purpose. In all the steam-engines that had hitherto been made the expansive power of steam alone had been made use of. Papin was the first person who turned to account its property of condensing, which he used to create a vacuum in the cylinder. He introduced a small quantity of water into the bottom of the cylinder; this he heated by a fire underneath, till it boiled and gave forth steam.

The steam, by its powerful expan

sion, raised the piston, which was at the bottom of the cylinder touching the water, to a considerable height, notwithstanding the pressure of the atmosphere on its other side. This done, Papin removed the fire, when the steam became condensed into water, and, occupying now about the seventeenhundredth part of its former dimensions, left a vacuum, through which the piston was carried down by its own weight and the pressure of the atmosphere.

Papin

The machine thus proposed by Papin exemplified two very important principles, neither of which appears to have been thought of in the application of the power of steam before his time. The first is, the communication of the moving force of steam to bodies upon which it cannot conveniently act directly, by means of the piston and its rod. The second is, the deriving of the moving force, not from the expansion of steam, but by the aid of its equally valuable property of condensing. did not work out his ideas, or indeed perceive their full importance, and he is believed never to have made an engine himself. Important as were the services which he rendered to the steam-engine, it is necessary, however, to observe, that it was the atmosphere and not steam which he employed as a moving power. It is true, that by the condensation of steam he obtained a vacuum, but upon this vacuum it was the atmosphere that acted, by pressing down the piston. Thus he overlooked in a great measure the expansive power of steam,

which had hitherto been the moving agent in the steam-engine.

Papin was also the inventor of the safety-valve, an ingenious contrivance, which, though not originally made for the steam-engine, was some time afterwards applied to it with great advantage. A valve, as most persons know, is merely a flap or lid closing some opening, and moving on a hinge. On one side, whatever presses against, or tries to pass through this lid, only shuts it all the closer; while on the opposite side, the smallest pressure against it instantly causes it to give way and open. Papin's safety-valve consisted of a valve closing an opening in the boiler. This valve was so weighted as to resist the expansive force of steam up to a certain point, when it was pushed open by the steam, which was thus able to make its 'escape through the opening in the boiler, long before it had acquired sufficient strength to burst the boiler.

Captain Savery, an Englishman, was the next person who turned his attention to the steamengine. The steam-engine which he made, though constructed on a very different principle to that of Papin, was possessed of considerable merits, and was the first real working engine that was ever made; but it is not necessary to give an account of it here.

We now come to the last improvement effected in the steam-engine before it fell into the hands of Watt. This was made by two Englishmen, Thomas Newcomen, an ironmonger, and