High idea that steam of high pressure possessed more advantageous properties than steam of low pressure gradually gained ground, and led to experiments with a pressure of from 15o to 30o pounds to the square inch. But the increased danger, the more rapid wear of the machines, and especially the considerable loss of steam by leakage, ren dered these experiments so unsuccessful that in practice there was only a gradual progress up to 45, 6o, and 90 pounds, and it is only in later prac tice that we find pressures of moo pounds and upward in ordinary use. Though with these higher tensions the condenser can be dispensed with, it nevertheless considerably increases the efficiency in cases where the expansive effect is to be turned to the best account, and as these cases occur more frequently the gain is considerable. Hence the endeavors to use a higher degree of expansion go hand in hand with the efforts to use higher tensions, so that at the present time the utmost is accomplished in this respect.
Rerab/ower's ComAound 1781, Jonathan ITorn blower, a contemporary of Watt, patented a "compound " or double cylinder engine (pr. St, 3) whose cylinders (A, h), which were of on equal sizes, were placed sick by side, while the piston-rods C, D of both were attached to the end of a beam overhead. Steam is led to the cylinder through the pipe G, Y. The cocks a, b, c, and d, which are adjustable so as to let the steam into and from the cylinders, are moved by the plug rod /1", which actuates handles not shown in the illustration. A' is the exhaust-pipe leading to the condenser. The cocks c, a being opened and b, d being closed, the steam passes from the boiler into the upper part of the cylinder 11, communication at the same time being opened betv:een the lower part of and the upper part of A. Before starting the engine the steam is shut off from the cylinder, which, by reason of the great weight of the pump-rod causes the pistons to rise to the tops of their respective cylinders. The engine being- freed of air by opening all the valves and permitting the steam to drive through the cylinders and out of the condenser through the "snifting-valve " 0, the valves b, d are closed and the cock ill the exhaust-pipe is opened. The steam beneath the piston of the cylinder A is immediately condensed, and the pressure on the upper side of the piston causes it to descend, carrying the end of the beam with it, and thus raising- the opposite end of the beam and its attachments. At the same time the steam from the lower end of the high-pressure cylinder is let into the upper end of the large cylinder A by the pipe 3; and the completion of the downward stroke finds a cylinder-fill of steam trans ferred frun one to the other, with a corresponding increase of volume and decrease of pressure. When the pistons have reached the bottoms of their respective cylinders, the valves at the top of the small cylinder B and at the bottom of the large cylinder A are closed and the valves c, d are opened. Steam from the boiler now enters beneath the piston of the
small cylinder, the steam in the larger cylinder is exhausted into the con denser, and the steam already in the small cylinder passes over into the large cylinder as the piston rises. Thus at each stroke a small cylinder fill of steam is taken from the boiler, and the same weight occupying the volume of the larger cylinder is exhausted into the condenser.
Lorpoid's —The high-pressure steam-engine—that is, all engine which is rendered effective without the assistance of atmospheric pressure—was first proposed by the prolific German technical writer Len poid in 1724, in his Thcatrum .1/adzinarum, from which our illustration I) is reproduced. It consists of two single-acting cylinders (r, s) which receive steam alternately from the same steam-pipe through a " four way" cock (.r), and exhaust into the atmosphere. The pistons c, d are thus alternately raised and depressed, which action raises and lowers the pinup-rods k, 1 by means of the levers i to which they are attached. The alternate action of the steam-pistons is secured by turning the cock x first into the position shown in the Figure, and then, at the completion of the stroke, into a reverse position, by which change the steam from the boiler a is led into the cylinder s, and the steam in r is discharged into the air. Leupold acknowledges his indebtedness to Papin for the suggestion of the peculiar valve employed.
Bull, in 1798, produced the Cornish single-acting pumping-engine without working-beam, the weight of the engine piston and pump plunger being carried by a weighted balance-beam. Oliver Evans intro duced the non-condensing high-pressure stationary engine which was the forerunner of most of our modern engines. Cugnot, Stephenson, and others applied the steam-engine to railroads; Stephens, Fitch, Evans, and Fulton, to steamboats.
Evans's 1779, Oliver Evans, an ingenious American mechanic, devised the first permanently successful non-condensing engine in which the power was derived exclusively from the tension of high pressure steam. In 1772, when but seventeen years of age, he turned his attention to the discovery of "some means of propelling land-carriages without animal power." Observing the power of steam exerted on a wad rammed down over a small quantity of water in a gun-barrel, from which, through the heating of the barrel in a blacksmith's forge, the wad was expelled accompanied with a loud report, he fancied he had discovered a new source of power. Meeting about this time with a description of a Newcomen engine, he was surprised to find that the elastic force of con fined steam was not there utilized, while the piston was moved by atmo spheric pressure. This he believed to be an erroneous application of the force of steam, and he conceived the idea of a high-pressure engine using steam at a pressure of about 120 pounds per square inch, which he pro posed to apply to the propulsion of carriages.