146

OPERATION OF ENGINE.

ing on an axis, and the rod of a pump or other weight was fixed to the opposite extremity of this balanced beam or lever*.

The space under the piston being filled with steam, and cold water applied to the sides of the vessel, the vapour, was condensed, and the column of air which rested on the piston pressed it to the bottom of the now empty cylinder. The fall of the piston depressing that end of the lever to which it was fixed, elevated the opposite end, and thus drew up the weight suspended from it. In this experiment the outline of the atmospheric engine was complete.

The power acquired by this means will be readily understood by a reference to the simplest experiments with the air-pump. A square inch of the surface of every body is pressed with a column of air which weighs about 14 pounds; a piston, therefore, measuring six inches on the side, or 36 square inches, is pressed with a weight of 531 pounds.

When the water in the boiler is heated to 212 degrees (Fahr.) the steam is capable of resisting this weight of air; that is, supposing a piston at the top of the cylinder, and the space under it filled with steam of 212 degrees, the piston would remain in its position; if the steam were raised to 214 degrees it would be raised out of the cylinder. But if the vapour beneath it was only 208 degrees, the piston would move downwards; that is," the elastic force of the vapour of this heat would be too weak to resist the weight of air pressing on the piston."

If, as has been already described, the steam in the cylinder is heated to 212 degrees, the pis

*See vignette, p. 139.

FRICTION OF PARTS.

147

ton will be supported by it, or prevented from falling downwards; but if cold water be applied to the sides of the cylinder, and the steam within it cooled, or condensed into water, it is obvious that the entire space which the steam occupied (deducting that which is filled by the water into which it has been converted) is a vacuous space, or the cylinder contains nothing able to resist the great weight of air laying on the piston; that body, therefore, falls into the cylinder in the same manner as a piece of iron weighing 531 pounds would do; and, by using proper intermediate means, is capable of performing the same effects that a piece of iron or other body of this weight can effect which falls from the same height that the piston fell in the cylinder.

That is on the supposition that the motion of the piston in the cylinder met with no retardation by rubbing or sliding along its surface, and that the balanced beam moved on its axis without experiencing any resistance; but in practice the case is widely different. Even when the inside of the cylinder is finely polished, and truly circular, and the sides of the piston made to work into the circular vessel with precision, and the balancedbeam to move freely on its axis, a considerable resistance is offered to the descent and rise of the piston, and to the vibration of the beam by friction. In this simple mechanism it would probably require forty or fifty pounds' weight to be hung at the extremity of the beam to raise the piston from the bottom to the top of the cylinder, supposing steam of 212 degrees was admitted to flow into the space as the piston rose. This power would therefore be considered as lost, because it is expended merely in moving the machinery; at this time another portion of the power

148

PACKING PISTON.

or weight of air, on the piston, was generally absorbed in consequence of a want of means to form the cylinder and piston perfectly true; and, when the vacuum was produced, the air insinuated itself into the spaces left by the inequalities of the work under the piston, and, of course, prevented its descent. The edges of the piston were therefore packed or fitted with some substance, which was elastic, and receded and expanded from protuberances, and into spaces, during the motion of the piston. Hemp was often used for this purpose; sometimes, say the authors of the time, leather was tried; with both tallow was used, as well to lubricate the surfaces as to fill up the pores of the packing, and a quantity of water was also allowed to lay upon the top of the piston.

The complete condensation of all the steam under the piston was another circumstance essential to the developement of the full power of the mechanism if this were not accomplished, the vapour that remained resisted the weight of the atmospheric column in proportion to its temperature. The effect of this was to diminish the amount of weight that could be suspended from

"About the year 1717, I communicated to Mr. H. Beighton, the use of the steel-yard over the puppet-clack or safety valve, which he applied to some engines. The way of leathering the piston was found by accident, about 1713: having then screwed a large broad piece of leather to the piston which turned up the sides of the cylinder two or three inches, in working it wore through, and cut that piece from the other, which falling flat on the piston wrought with its edge to the cylinder, and having been in a long time was worn very narrow, which being taken out, they had the happy discovery, whereby they found, that a bridle-rein, or even a soft thick piece of rope or match going round, would make the piston air and water tight." Desaguliers' Experimental Philosophy, p. 333. vol. ii.

the pump-end of the beam; that is, if the vapour remaining in the cylinder had a temperature which would resist a column of air equal to three or four pounds on the square inch, so much, of course, was deducted from the total pressure of air on the cylinder. The air itself had also an effect in preventing the fall of the cylinder, even when it was prevented by the usual means from finding its way into the vacuum between the sides of the receiver and the piston. The water from which the steam was generated contained more or less air, which boiling disengaged; this entered with the vapour into the vacuum; the steam with which it was mixed could be condensed, but air could not, and it remained in the cylinder, and prevented, by its greater or less action, the fall of the piston, by filling the space under it: from this cause alone, (and which was afterwards known by the term of wind-logged,) this engine must have soon ceased its motion. This defect could seldom be very apparent in Savery's apparatus, and a small quantity of air always laying in his receivers would be rather an advantage than otherwise, as it would prevent the cold surface of the water from condensing the vapour; and, lastly, the new form of the steam cylinder, which now required to be open at the top and perfectly cylindrical, and placed perpendicularly, was exceedingly unfavourable for permitting the vapour to be condensed, as Savery did it, by an affusion of cold water on the outside of the vessel; indeed his scheme would have been nearly impracticable with Newcomen and Cawley's appa

ratus.

The defects of this rough experiment have been enumerated at a greater length than their importance may seem to demand, but as it is only in the first stage of the invention that the steam cylinder

150

SAVERY'S CLAIM.

of Papin can be brought into the comparison, it appeared but fair that the peculiarities of each should be even amply displayed: for on the whole, although Papin may be certainly considered as the author of the mode of elevating a piston by steam, and afterwards condensing the vapour, his labours beyond the first step could offer but small assistance to Newcomen and Cawley.

The difference that the use of a lever probably made in the experiment of Newcomen has been already stated. The pipe by which Papin supplied his cylinder with water, also served as a pipe by which the air which was disengaged from the water escaped from under the cylinder.

These ingenious persons had, however, a rival to propitiate, who claimed the invention of a part of the mechanism* ; and the individual who urged this claim, was not only a man of enterprise, but had both the influence and means to maintain his pretensions; and without his concurrence, Newcomen and Cawley found it impossible that their scheme could ever be carried into practice-Captain Savery was in possession of the right to use the method of making a speedy vacuum by condensing Steam; and in the grant of an exclusive privilege

"As the best and most useful inventions and improvements, which have been discovered either in art or nature, have in the process of time been subject to the same, so this ingenious gentleman, (Mr. Newcomen,) to whom we owe this late invention, (the Fire-engine of Savery,) has, with a great deal of modesty, but as much judgment, given the finishing stroke to it. It is, indeed, generally said to be an improvement on Mr. Savery's engine; but I am well informed that Mr. Newcomen was as early in his invention as Mr. Savery was in his, only the latter, being nearer the Court, had obtained his patent before the other knew it; on which account Mr. Newcomen was glad to come in as a partner to it."-Vol. ii. p. 216., Switzer's Hydrostatics,