Rotatory Steavi-Engine. 55 



more difficult of construction than a straight, round cylinder and 

 piston, which, being derived from the straight line and circle, are 

 the simplest of forms, while an annular chamber, if the piston 

 be rotatory, is a surface of double curvature, of difficult con- 

 struction and imperfect completion ; if square, the construction 

 of a rectangular piston is a still more troublesome attempt, the 

 increased surface being increased expense and labour. 



II. In point of bulk, the common reciprocating piston has a 

 decided advantage ; the annular cylinder of the revolving piston 

 must be (to give equal power) about two-thirds of the area, and 

 about three times the length of the reciprocating cylinder, being 

 a bulk of cylinder nearly double. But even this is an estimate 

 too favourable to the piston of rotation ; the diameter of the axle 

 requires to be very considerable ; there are various reasons for 

 this, — one is, that it is frequently a steam-passage ; another isj 

 that it is much larger than is required for the mere purpose of 

 communicating the force, because any force of steam applied 

 near the centre is of little value in producing an effect, from 

 the smallness of the circle which that part of the piston de- 

 scribes ; and for this reason also, that the portion of piston ex- 

 posed to leakage and wear is in proportion to the effect gained ; 

 the piston is therefore removed to a considerable distance from 

 the centre, to answer the purpose likewise of rendering the re- 

 volution of the parts more nearly equal. These points will, 

 however, have our attention at another time ; it is sufficient for 

 our present purpose, that these cii'cumstances render it impe- 

 rative to increase the bulk of the engine. 



III. In point of durability and economy in its use, the nxost 

 conclusive arguments lie against the rotatory engine. I have 

 seen many of them perfectly constructed, working beautifully, 

 but they went very soon out of order. They invariably work 

 best when new. This may appear to some to arise only from 

 the defects incidental to particular modes of construction. I 

 admit that many have had peculiar elements in their construc- 

 tion not indispensable to the principle. But, on the other 

 hand, I shall now go on to shew, that, independent of the idio- 

 syncracy of peculiar engines, the necessary mode of action in- 

 volves elements of self-destruction very rapid in their operation, 

 by means of which every rotatory piston must soon wear itself 

 out of condition. 



