It has been mentioned that there is a limit to the proportion between the diameter and length of the cylinder ; the advantage that would accrue in gain of power by a long stroke being diminished by the greater radiation of heat from the larger surface diminishing the force of the steam in the cylinder ; here therefore, as in every other calcula tion connected with the steam-engine, it is hardly possible to arrive at any formula or rule that can be invariably used If the surface of the cylinder were to be made a minimum, with a maximum of capacity, we could readily determine that the length should be twice the diameter ; * but we find that this proportion is not adhered to by the best makers; it varies from 3 : 1 to 2 : 1 ; but in the marine engine it is nsually shorter ; in some instances the proportion is 1 : 1.25.
The diameter of the cylinder of a marine-engine is usually greater, in proportion to its length, than it is in others, in order to obtain, by an increased surface of piston, that power which is unattainable by a long stroke, owing to the limited space which can be appropriated to the engine. Formerly, the apprehension of danger, where so many lives were at stake, prevented the use of steam of more than 4 to 6 lbs. on the inch in marine engines, and the expansion principle conse quently could not be had recourse to. At present, the economy of using this principle has ontweighed the apprehension in the minds of the owners of vessels, while the public, contented with the iufortnation that the engine is a condensing one, seldom inquire further, and con ceive that the steam is at a low pressure in all marine engines, although, where the expansion principle is used, which it now extensively is, the pressure in the boiler is at about 30 lbs. on the inch above the pressure of the atmoaphere.+ 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 set directly to produce such a motion. It does indeed appear at first sight that, where the object of the engine is to produce a continuous circular motion of a fly-wheel, or of wheels of seine kind, it would be desirable that the steam should be applied directly to impel the wheel, instead of having its force trans mitted through a series of levers, necessarily increasing the friction and the cost of the engine. Watt accordingly patented more than one of such rotatory engines, and many others since have from time to time brOught forward arrangements for the purpose, but none have come into permanent and general use. The fact is that, as can be easily shown, 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. In all rotatory steam-engines hitherto proposed, the principle has been to admit the steam to act on a fan or fans revolving round an axis of the cylinder ; and, by ingenious excentric movements, the surface of these fans is made to increase as the steam diminishes in elastic force from the enlargement of the space it occupies. Many such engines have been used for a
time, but commonly 'after a few years' trial they have been abandoned, and the reciprocating principle substituted, thus proving that experience confirmed the deductions from theory.
In all mechanical combinations, the object to be effected necessitates a certain characteristic form of the machine, which it retains, whatever improvements may have been successively introduced either in its principle or in the details of its construction. We can recognise in a modern Sussex plough the general form of that used by the subjects of the Pharaohs to till the banks of the Nile ; and Newcomen would acknowledge a marine engine made by Maudslay and 'Field as a descendant of his atmospheric one : but he would for some time be at a loss to tell the object of a locomotive engine of Stephenson's, if he could see it at rest only, and the connection between it and its tender would be beyond his comprehension. The reason of this is, that a locomotive engine is a perfectly new application of the power, having no other analogy to an ordinary engine except that steam is the source of power in both ; but all locomotive engines will hereafter possess a family likeness.
The principal causes of this novelty of form are, that great velocity being the.object, the boiler must constitute the greatest part of the bulk, in order to supply a sufficient quantity of steam to meet the demands of the two pistons making 200 strokes per minute, and even then it requires a tender to accompany it to carry a supply of fuel and water to keep up the quantity. The locomotion is usually produced by a pair of driving-wheels made to revolve by the engine by means of cranks on their axle, receiving motion almost directly from the piston rods ; the adhesion which takes place between these wheels and the edge-bars or rails, causes the carriage to move on, just as the paddle wheels of a steam-boat propel the vessel by the resistance of the water to the float-boards. The driving-wheels of a locomotive engine do not, any more than the paddle-wheels of a steam-boat, act as fly-wheels to regulate the velocity of the engine ; this is effected in the former by the inertia of the whole masa; the great velocity consequently ensures the steady action of the engine. In the steam-boat this great velocity of the paddles is unattainable, and consequently two powerful engines are requisite to maintain even a moderate velocity; but air being the medium in which the driving-wheels of a locomotive engine act, almost any velocity can be imparted to them, and for obvious mechanical reasons is beet attained by steam acting with a force almost amounting to impact ou small pistons through a short stroke, two alternately acting cylinders being necessary, as in the steam-boat, to equalise the action of the cranks.