When Watt substituted the elastic force of steam for the pressure of the atmosphere, he introduced a source of power which might be increased to an indefinite extent, provided it were found advantageous to employ it. Gene rally it is more advantageous to employ steam of a comparatively high elastic force ; accord ingly the pressure was increased, in engines constructed by Watt, from 4 to 8, or even 12 lbs. on the inch.. Modern boilers are so skilfully made, that steam of 100 lbs. on the inch may frequently be safely used. In the common engine, if the pressure on the piston continue uniform during the stroke, as it would do if the communication with the boiler remained open, the piston would move with an accelerating velocity till it arrived at the end of the cylinder, by which it would produce mischief; hence the steam is cut off at a particular stage in the descent ; and when the steam possesses considerable elastic force, the communication with the boiler is cut off sooner, and the piston is urged forward by the expansive force of the steam, 'which, although decreasing as the space increases, is yet sufficient to carry the piston to the end of the stroke.
Although the Condensing Engine is very perfect and economises fuel, it is necessarily heavy, and occupies considerable room. For cases where the two last-named particulars are of importance, the Non-Condensing Engine was invented. A locomotive for railways is the most important form of non-condensing engine ; but such engines are frequently valuable for other purposes. Since the pumps of the condensing-engine are dispensed with in the non-condensing engine, the beam may be so likewise ; and a still further simplification results from an oscillating movement given to the cylinder. Such engines are termed Vibratory Engines, and are successfully used where space must be economised, as with marine engines. Non-Condensing Engines are frequently termed High-Pressure Engines, in contradistinction to Low-Pressure or Con densing Engines.
Marine Engines, or those used for propelling vessels, are in this country generally con densing engines, their situation admitting the abundant use of cold water. The principal peculiarity in the arrangement of the marine engine is the position of the beam, which, for the purpose of economising room, is placed lower than the cylinder, and is double, there being one on each side. In all vessels of any magnitude, there are two engines complete, so arranged, that while the rod and crank of one are in their neutral position, those of tho other are in that of greatest effect. In steam vessels of smaller size, the direct action is often used ; that is, the piston-rod turns the shaft without the intervention of any beam. In American steam-vessels the engines are frequently high-pressure.
Engineers have always been induced, by the obvious advantage of a continuous over an alternating motion, to aim at contriving a steam-engine in which the steam should act directly to produce such a motion. Watt
accordingly patented more than one of such Rotatory Engines, and many others since have from time to time brought forward arrange ments for the purpose ; but none have come into permanent and general use. The fact is, that the employment of steam in this way is productive of a greater waste of power, with a greater increase of friction, than can be com pensated by any real advantages.
The peculiarities of the Locomotive Engine are noticed under RAILWAY.
No part of a steam-engine exhibits more variety, according to the purpose to which it is to be applied, than the Boiler. Rapid gene ration of steam, .security, compactness, and lightness, must be aimed at in boilers for marine or locomotive engines, even at the cost of a comparative waste of fuel ; while for those intended for pumping or driving machi nery, economy of fuel must be the paramount object, the weight, form, and space occupied by the boiler being secondary considerations. As very large boilers are unsafe, it is custo mary to use two or more small boilers instead of one large one ; and the principle, carried to its limit, constitutes that of the Tubular Boiler, in which the steam is generated in a series of independent metal pipes of small diameter, all communicating with a. common steam-chamber, or reservoir, itself small, and strong enough to resist great pressures. The safety valve is a contrivance for letting off some of the steam from the boiler when the elasticity is dangerously high. Gauges are employed to measure the quantity of the water, and the pressure of the steam in the boiler.
Whenever the steam-engine is employed to execute any work which is variable in its quantity or intensity, there must be some means of adjusting the force of the engine to this varying resistance it has to overcome. The object of that beautiful piece of mecha nism termed the Governor is to enable the engine to regulate the supply of steam admitted to the cylinder, or in other words to adapt the force it has to transmit to the resistance it has to, overcome. In this appa ratus, two balls rotate round a common axis, with a velocity depending on that of the fly wheel ; if this velocity is great, the balls divergrby virtue of the centrifugal force, and this divergence is a means of partially closing a valyp, and lessening the amount of steam which enters thp cylinder. From different causes the marine engine and the locomotive do not require governors.
A museum of steam-engines would display some of the highest results of human inge nuity ever manifested. Among the toys or curiosities of such a museum might be placed the tiny steam-engine, made by a mechanic of Saddleworth, for the Great Exhibition, and weighing only three-quarters of an ounce.