The advantage of this method of working a steam-engine increases in portion, as the steam is sooner stopped to the extent indicated in the annexed (marginal) table. Let the steam be stopped at either of the depths of the cylinder stated in the first column, and its performance will be multiplied to a degree corresponding with the figures placed opposite to each division. This advantage, also, is more fully obtained when steam of an elastic pressure considerably greater than the atmosphere is employed ; the increase of the force of steam heated much. above the boiling point being rendered very great by a small increase of heat; "thus, at 212° the steam is equal to the atmosphere ; by only increasing the heat 40°, or to about 252°, a double pressure (or two atmospheres) is obtained ; and the further increase of 30°, or, about 282° makes an additional force equal to three atmospheres ; ain 24° higher, or about 306% makes the steam equal to four atmospheres;ill e law in this respect has been since determined with precision. See page 737.] Now T it follows as a consequence, that if such small accessions of heat produce so rapid an increase of the expansive force, small abstractions of heat from highly elastic steam will also reduce its elasticity in an equal degree ; so that steam highly heated is more readily diminished in bulk by the application of cold than weaker steam ; that is, it can be more readily reduced in its pressure to , any certain proportion of the pressure it had before." fo obtain the full advantage of the varying forces, it was necessary to have sonic contrivance by which the effect of the engine on the work it is performing should be uniform, or nearly so, and Mr. Watt devised many different methods for We purpose ; but they were never applied to his engines to any notable extent ; contenting himself with employing steam a little greater in pressure than that of the atmosphere, and stopping the supply at one-fourth or one-third of the descent, according to the circumstances under which the engine worked. And though the curious and important facts thus elicited by Mr. Watt's endeavour" to economize steam were not made extensively subservient to his uses, they paved the way to new improvements at a subsequent period by other persons, which will be noticed and illustrated in the proper place.
The enormous loss of power occasioned by extensive friction, by the alter nating action of the engine, and by the wear and tear of its parts from and other causes, became apparent to him at an early period of his pursuits ; and, whilst still engaged in the perfection of his machines constructed upon the principles before described, he had diverted his speculations into a new channel, in order to discover a means by which this loss could be repaired. The patent granted to him in 1769 includes the description of a mode for obtaining the effective power of steam unburthened by these deteriorations, namely, by giving " circular motion to a wheel,"or in other words, a Rotary Engine ; but from the obscurity of the specification on this head, which rather suggests than explains such a contrivance, it is very evident that little more than a crude conception of its structure had at that time occurred to his mind.
[It is remarkable enough that no drawings accompany the explanation of this, or any one of the six inventions included in this famous patent. Mr. Watt has been a good deal assailed on account of the vagueness and ambiguity. in which all his descriptions are involved, and which harsh commentators scruple not to aver was premeditated. The very natural desire, in a mind teeming with speculations, and confident in its own resources, to take first ground, superadded to the commercial shrewdness for which his countrymen are proverbial, may possibly explain this over-cautious effort at self-protection.) By his own statement it would appear, that "a steam-wheel moved by force of steam acting in a circular channel against a valve on one side, and against a column of mercury or other fluid metal on the other side, was executed at Soho, (the manufactory of Bolton and Watt at Birmingham,) upon a scale of six feet, and tried repeatedly, but was given up, as several objections were found against it. What these objections were, is not declared ; but this part of his patent wail
of no value beyond identifying his name as the first suggestor of a Engine. Undisheartened by the first failure, he again, in the same year, 1782, obtained a second and distinct patent for -two other devices on a similar principal. These we will shortly descnbe.
c c, in the preceding figure, represents a drum or cylinder, of any given dimen sions, say a foot deep, and three feet in diameter. a, an axle passing through stuffing boxes in the closed ands or coven of the cylinder 6, the piston packed at the ends which rub against the cylinder, and at the sides which rub agqnst the covers, to render it steam tight ; this piston is inflexibly attached to theaxis a. e, a valve or flap turning upon a joint or the concave side being a segment of a circle of the same radius as the cylinder. It extends the whole depth of the cylinder, is packed on its sides, and, when shut, fills the space at d. g, the pipe for admitting steam from the boiler ; A, the pipe for its escape into a condenser. Steam being admitted through g, presses equally upon e and b ; but b being stopped against the axle, the piston 6 recedes from the pressure, and turns the axle a and • heavy fly-wheel round with it. This motion is continued until the convex surface of b comes in contact with the concave surface of e, and, by the momentum thus acquired, forces e into the recess d, until the piston again passes', when the valve, falling by its gravity (or moved by a lever) into its first position allows another portion of steam to enter and act as before, the steam contained in the drum passing off at the eduction pipe A.
It will be readily understood that the enormous waste of steam hi working this engine, the shocks caused by the motion of the valve, and the facility of derangement generally, are capital objections to this device, which became as speedily abandoned u the first. The third contrivance, however, was very much superior.
The cylinder d d is similar to the last, the interior of which is shown in the cut: a the axle, to which is attached the piston b, as before, made steam-tight by packing. c a metal projection from a portion of the cylinder to the axis. e f two valves, to admit steam from the pipe g into the cylinder on each side of c alternately. o f valves for changing the direction of the steam ; i j valves acting in conjunction with ef, so as to open or shut off a communication with the con densers 11c, through the pipe A, at the proper time. Levers are attached to the rods, by which these valves are worked, from tappets on the pump-rods r g. Steam being admitted from the boiler through the pipe g into the steam-chest, the valve/ being open, rushes nu the Wipe, and into the cylinder, between the piston and stop c. Tice piston receding from the pressure drives the fez in the cylinder through the other pipe, and down through the valve) into the con denser, whence it escapes by the pump L The revolution of the piston con tinues until it reaches c, on the other side, when it is stopped ; but, previously to this, the valves f andj have been shut by their respective levers, whilst e and i have been opened. The steam has now access through e to the other side of the piston, and turns it in the contrary direction ; the steam which last per fbrmed its office escaping down through i to the condenser. The first operation is then repeated, reversing the motion of the piston as soon as, or before it tomes in contact with the other side of e. n in are two toothed wheels attached to the axle a, which work (as shown) by racks, the op, and the smaller pump-rods q r. The former, o p, are to draw water from a mine; the smaller ones only work the condensing pumps k 1.