The two internal cylinders are most conveniently placed under the boiler in nearly a horizontal position ; intermediate cranks preserve the rectilinear motion of the piston-rods, end connect them with the cranks on the axle ; while excentrie frames on this axle actuate the simple slide-valves required in a non-condensing engine. The steam which is driven out of the cylinder at each stroke, instead of escaping directly into the open air, passes into the funnel of the furnace, and thus increases the draught ; but as the action of the engine cannot be understood independently of the boiler, it becomes necessary to turn our attention to that subject, without a general description of which, any account of the steam-engine would be incomplete.
Upon the South-Western Railway of England, and on the Chemins de Fer de l'Oueet of France, the engines employed arc made with external cylinders, and the piston-head is connected with the crank fixed on the driving-wheel by a link or connecting-rod. There are great advantages in this system, so far as economy of construction is concerned ; for the cranks are simply keyed on the ends of the axles of the driving-wheels, instead of being forged upon the axles themselves, and the whole of the machinery is of a simpler and more efficient character ; but at the same time the condensation in the cylinders is far more serious than it would be in the case of internal cylinder engines, on account of the more exposed position of the external cylinder engines. The latter engines, it may be added, present the advantage of having a smaller dead weight, per horse-power exercised, than is the case with internal cylinder engines.
Since some of the principal objects to be kept in view in the con struction of boilers are incompatible with each other, one or more must be dispensed with in order to secure the rest. The specific purpose for which the engine is constructed must determine the general prin ciple of the boiler : thus, rapid generation of steam, security, compact nese, and lightness must be aimed at in boilers for marine or locomotive engines, even at the coat of a comparative waste of fuel; while for those intended for pumping or driving machinery, economy of fuel must be the paramount object, the weight, form, and apace occupied by the boiler being secondary considerations. And whenever steam of a high elastic force is to be used, that form of boiler should be adopted which will most nearly equalise the strain on it. The following requisites are therefore to be understood as being those which it would be desirable to combine, though, united, they are unattainable in practice.
First, the boiler should have the greatest capacity with the least surface, to save material, diminish the weight, and increase the strength : hence a spherical form would be best in this respect, but it is incom. patible with an economical application of the heat to a great extent of surface, which is essential to the rapid generation of steam.
Secondly, the form should be as simple as possible, both for the sake of reducing the expense of construction and most readily admitting of repair. Boilers are made of iron or copper plates riveted together at their edges ; and if one of these is cracked, or has been burnt, that plate can be taken out and another put in without pulling the whole to pieces, which must be done when the boiler is of a complex form ; added to which, all angles are sources of weakness, owing to the inequality of the strain on the adjoining surfaces, and the injury done to the metal by bending it to form the angle.
The form of boiler used for Savery's, Blakey's, Newcomen's, and other engines of the 17th and 18th centuries, up to the time of Watt, was that of an inverted frustum of a cone, with a spherical top, and its bottom slightly concave. This boiler was set in brickwork like a common copper, the flame playing round the whole of the lower part. The steam-pipe was connected in the usual way to a flange of a collar in the spherical head. Watt adopted a long rectangular form, with a semi-cylindrical top; the ends were flat and upright, the sides slightly curved inwards, as was also the bottom. From this form it is termed the waggon-head boiler. It is set in a rectangular mass of masonry, the cylindrical head alone projecting above the level of the brick-work the fire-place was underneath one end of the boiler, and extended backwards for one-third of its length ; the flue, after proceeding to the further end, returned along one aide, across the end, over the furnace, and along the other aide into the chimney-abaft, the boiler itself every where forming one side of the flue, and consequently having the flame and heated air directly in contact with it at the bottom and sides. In some cases, when the boiler was very large, a cylindrical iron flue was formed through the boiler longitudinally, opening at each end into and forming a continuation of the brick one, thus increasing the surface to be acted on by the heat.