The " beam " so frequently alluded to, was obviously the readiest mode of connecting the alternating motion of the piston with the pump to bo worked, in the atmospheric engine; and owing to the facilities it offers of working the plug-tree and the three pumps neces sary in Watt's condensing engines, continued to form a part of the arrangement whether the engine were intended to pump a mine or to drive machinery. The beams of the first engines were made of two or more trees, bolted together to obtain the requisite rigidity, and further strengthened by a kind of truss, as is seen in the diagram of Newcomen'a engine. But when the art of making heavy Iron-casting, was perfected, that metal was substituted for wood, to the manifest improvement of the engine in every respect. Watt also removed the cumbrous arched heads, which had been previously employed for the purpose of causing the piston-rod to move up and down in the same right line, though connected with the end of the beam, which neces sarily described an arc of a circle, as turning on a fixed centre; this arrangement implied the use of a flexible chain, to suspend the piston, which might wind round, and unwind from, the arch, but a chain * The term stroke is technically used by engineers to express the whole motion of the piston from the top to the bottom of the cylinder and back again.
could not be used when the piston had to raise the beam, as it had to do in Watt's engine, instead of being raised by it, as in Newcomen's. The object of these "arched heads" is attained in modern engines by a system of simple rods or levers, so combined that one point may move in a straight line nearly. There are a variety of combinations by which this may be effected, but that termed the " parallel motion," invented by Mr. Watt for the purpose, is the only one which need be here noticed, as being that most commonly used.
The geometrical principle of this motion is shown in figs. 9 and 10 as well as the whole arrangement when the piston is near the top of the cylinder n D are rods fixed by one end to the frame supporting the beam, while the three other pair of levers being jointed together and to the beam, must obviously, in every position, form a parallelepipedon, whence the name is derived ; r is the piston-rod attached to ; ct that of the hot-water pump connected with the parallel motion at t in the centre of that side.
When the engine is employed to drive machinery of any kind, a fly wheel becomes a necessary adjunct to it. A fly-wheel is one in which the principal quantity of the matter is distributed in the periphery; when such a wheel revolves on an axis perpendicular to its plane, the greatest quantity of matter moving with a maximum velocity, the momentum of the whole is a maximum, while its inertia, if it be large, causes it to control, or equalise, the motion of the machinery through which it receives its own. It is the momentum of such a wheel which
constitutes the disposable force available for the multifarious purposes to which machinery can be applied ; so that in the case of the steam engine, although the elasticity of steam is the original source of power, the immediate one by which the work is executed is the momentum of the fly-wheel.
It is consequently necessary to adapt some contrivance to the end of the beam, which shall convert the alternating circular motion of the latter into a continuous one of the flywheel; this is effected by the rod and crank, a piece of mechanism of such frequent occurrence that it is unnecessary to describe it; the treadle of a lathe is a familiar instance of its application, and for a similar purpose, that of connecting the alternate motion of the turner's foot with the continuous one of the wheel of the lathe; the principle of the treadle, or rod and crank, is in fact the only one by which an alternating can be converted into a continuous circular motion ; it must therefore be employed, notwith standing the variation in the power transmitted by means of it, conse quent on that of the angle formed by the rod and crank with each other. Thus, for example, when the rod and crank are in the same direction, which occurs twice at every rotation, no force whatever is transmitted by it, and the primary one is entirely suspended or held in equilibrium by the resistance of the fixed centres on which the crank and rod turn.
In the steam-engine the rod and crank are so adjusted that these two neutral positions occur when the piston is at one or the other end of the cylinder, and the valves are so arranged that, both steam passages being closed, all communication between the engine and the boiler is cut off; otherwise the steam, which could not under these circum stances move the piston, would exert its force to the detriment of the machine; as soon, however, as the momentum of the fly-wheel has carried the crank past these positions, the motion reciprocally imparted through it to the piston and valves admits the entrance of the steam from the boiler into the cylinder again. It is one of the important details in the construction of the engine, that the piston should be in that point of its course when the steam exerts its maximum of effect on it at the time when, the rod and crank being at right angles ' to each other, the maximum of force may be exerted to turn the fly-wheel.