1844. 



THE CIVIL ENGINEER AND ARCHITECrS JOURNAL. 



'^oo 



dent, delivered a lecture ' as it was called, but which was rather an 

 oration in praise of the art, and indeed with a little variation of names 

 it would answer equally well for an oration in praise of any thing else. 

 It had little which was peculiarly applicable to the art, not a single 

 drawing nor model was exhibited. Many things were stated which 

 were perfectly true, but nothing that was at all new; and we much 

 donbt if any of the small, but respectable and attentive, audience have 

 been more fortunate than ourselves in carrying away a single new idea 

 on the subject. It put me in mind of the late Lord Casllereagh's 

 speeches in the House of Commons, all he said appeared to be to the 

 purpose, but the moment he ceased speaking it was impossible to re- 

 collect anv thing that he ha.l said. Still the Vice President is en- 

 titled to the thanks of the public and the profession for breaking the 

 ice, and setting an example which he intimated would be followed up 

 by others of the Institute. The only point at all forcibly urged (and 

 vjith which we fully agree), was that proper edifices could only be 

 built by properly qualified persons, and that it was not proper to put 

 architects' plans into the hands of builders. He also stated "that a 

 few but surpassingly beautiful edifices have been erected in Dublin," 

 but to our great regret omitted to mention where we could find them 

 out. A high encomium was passed on the late President Lord Vesey 

 Fitzgerald, but we do not recollect any mention being made of our 

 own proper exertions to bring into notice the beautiful facade of his 

 Lordship's Antrim house, see page 97 of this volume. 



Clonmon, Dublin, 

 July, 1844. 



1 A copy of the oration has come into our poaaesBion, which we have giren in another 

 part of the Journal. 



ON THE GENERAL PRINCIPLE OF ROTARY ENGINES. 



There has been a prevalent opinion from the time of Watt to the 

 present, that if any grand improvement of steam engines be intro- 

 duced, it will be effected by causing the steam pressure to produce 

 rotatory motion immediately nithout the intervenlion of a crank. This 

 opinion has however received great discouragement from the high 

 authority of Tredgold, and in his condemnation of rotatory engines, 

 he has committed in one instance, such an important error in mechani- 

 cal principles, that attention ought to be called to it. 



Throughout his great work on the steam engine, Tredgold excuses 

 his brevity in noticing various rotatory engines on the score of prac- 

 tical objections, such as leakage, friction, and the difficulties of con- 

 struction. Not content however with these 7?rac^/ca^ objections, he 

 attempts to prove theoretical objections also ; these are now to be 

 considered — of the practical objections no more need be said at pre- 

 sent, than that he seems to have overrated them, owing to his sup- 

 posed discovery of an error in the first principles of rotatory engines, 

 and that his objections after all can only extend to those rotatory en- 

 gines known and examined by himself. It seems impossible for him 

 or any one else to predicate a priori practical objections to an unex- 

 amined engine. The same objections have been made long ago to 

 reciprocal engines, and arguing abstractedly, there is no reason on 

 earth for assigning greater difficulties to rotatory than reciprocating 

 engines. With regard to the practical difficulties, then, experience 

 can alone guide us ; all we can safely say is, that in all rotatory en- 

 gines Ai/fcrto known, great practical difficulties do exist: this does 

 not affect future discoveries. 



With regard however to Tredgold's theo- 

 retical objections, something more may be 

 said ; because if those objections were cor- 

 rect, they would affect not only all present, 

 but also all future constructions. His ob- 

 jection, which is not expressed very clearly, 

 is this — " The pressure on an inch at the 

 most distant part C, from the centre of mo- 

 tion is the same as the pressure on an inch 

 at the part E nearest that centre. But since 

 the piston is constrained to move in a circle, 

 the effects of these equal pressures are as their distances from the 

 centre of motion, and limited by the effect of the pressure at the most 

 distant part C. If the centre of curvature were nearer the side of the 

 vessel, the effect at E would be less. Therefore the effect of the 

 pressure to produce motion is less than in a straight vessel having the 

 same base ; and if the bases be the same, the space the pressure acts 

 through will be as the quantity of steam, consequently the quantity of 

 steam being equal the power of rotatory action, will be less than that 

 of rectilineal action." P. 158, sec. IV. 



He then proceeds with an analytical investigation, and concludes 



the power in the rectilineal engine to be greater than that in the rota- 

 tory ; his reasoning is however vicious in principle. 



This error in the analytical investigation cannot be better explained 

 than in the words of his own Editor Woodhouse, whose observation, 

 being among the errata of the work, has frequently been overlooked. 

 "Tredgold has here made use of the moment of the pressure of the 

 steam, instead of the actual pressure on the parts. The force at the 

 distance x from E >s/y, d x, which multiplied into 2 v (r -\- x), the 

 space described, gives 2/' ir )/ d x ()■ + *); and the integrate xs/t a y 

 (2 r-|- a), the power expended at each revolution. It is hence plain 

 that no power is lost, the factor -a a y [2 r -\- a) being the volume of 

 steam employed at the pressure/, risthe distance D E, 2/ the breadth 

 of the piston, y the pressure on a square inch. 



The above calculation by Woodhouse, shows the quantities of power 

 exerted by equal quantities of steam are the same in rotatory and re- 

 ciprocating engines. To understand the important error of Tredgold, 

 it may be observed that the " moment of pressure," is " the pressure 

 multiplied by the length of leverage by which it acts," and this quan- 

 tity Tredgold has mistaken for the pressure itself. To explain, 

 without mathematical reasoning, we must consider what is meant by 

 the phrase " loss of power." It does not mean that of pistons of equal 

 size, tl'.e reciprocating piston will produce most force. This is not 

 the question — it is rather whether, when equal quantities of steam are 

 employed, the reciprocating engine has most force. This distinction, 

 simple as it is, explains the whole difficulty. The pressure on an inch 

 at E in the above figure, has certainly less force than pressure on an 

 inch at C, and if E were very near the centre D, the pressure at E 

 would produce scarcely any effect whatever. To compensate this, 

 however, the parts at E describe less space in their revolution than 

 those at C, and consequently require less steam. And it is found by 

 mathematical investigation that this compensation is so exact, that 

 •vihete equal quantities of ileam are employed, the force is exactly the 

 same in both kinds of engines; in fact this conclusion might have 

 been suspected beforehand, by considering that if power were lost, it 

 must have expended itself somewhere, and this could only be by one 

 of these two ways — either by the escape of the steam, or by its being 

 employed to produce motion in something else besides the piston — 

 both which suppositions are excluded from the investigation. 



There is however another way of viewing the matter in which there 

 is a positive gain of power on the rotatory system. It is known that 

 in all reciprocating engines the number of strokes per minute is 

 limited, and the larger the piston, the greater the difficulty of increas- 

 ing the number of strokes. This arises from the momentum acquired 

 by the piston's motion in one direction having to be destroyed before 

 it can return. The direction of the motion is constantly changing, and 

 it is obviously more difficult to make a large body move up and doimi 

 rapidly, than continue the motion always in one direction. Hence 

 arises the well-known contrivance of cutting off the steam before the 

 stroke is completed : if the pressure were not stopped, the piston 

 must violently strain the connecting crank, or must be met by a great 

 pressure of steam on the side opposite. Now although by cutting 

 off the steam, no power in one sense is lost, because no steam is lost, 

 still there is a loss of time, owing to the steam being unemployed 

 during part of the stroke, and the rapidity of the engine is conse- 

 quently limited. In a rotatory engine however, the motion would be 

 always in one direction, and would be constantly accelerated by the 

 steam. 



There are other advantages which may be expected from a practi- 

 cable rotatory engine. It may be concluded from these already pa- 

 tented, that such au engine would have little external machinery, that 

 is, all the moving parts would be contained in the steam chamber con- 

 taining the piston. Hence might be expected these immense advan- 

 tages — the great diminution of the weight and bulk of the engine — 

 and consequent on the simplicity of the mechanism would be the 

 facility of making it, and the diminution of the danger of getting out 

 of repair. 



When the great desideratum of a practicable rotatory engine has 

 been accomplished, a new era in the history of the steam engine must 

 commence. Correct theoretical principles must prevail when the 

 practical difficulties have been overcome. That there are no theore- 

 tical disadvantages may be mathematically demonstrated — of the 

 practical difficulties it is illogical to \txeA\c3.\,e generally. It is a com- 

 mon custom to do so, because rotatory engines already tried have 

 failed. But until some proof can be shown of the necessary difficulties 

 of the rotatory principles, this condemnation must be considered an 

 unreasonable generalization. There were full as many in the veay of 

 reciprocating engines, but the perseverance of Watt did that for re- 

 moving the practical difficulty which his genius did for removing the 

 theoretical. H. C. 



22* 



