30 THE HISTORY OF [SECT. i. 



The effect would be nearly the same as that derived from cutting off the steam 

 before the piston arrives at the end of its stroke, as was afterwards done by 

 Mr. Watt, (art. 28.) but has the decided advantage of being a more equable 

 method of employing steam power ; and in large engines the construction of Horn- 

 blower's is also superior, because strong steam can be used in a small cylinder with 

 less risk : he however does not appear to have intended to use powerful steam ; 

 and he could not use his invention, because the improved mode of condensation 

 remained the right of Boulton and Watt. 



Like Watt, many other engineers imagined that a rotary motion might be com- 

 municated with advantage by the direct action of steam ; and two combinations 

 for that purpose were made by Hornblower. His first was an ingenious but com- 

 plicated machine, for which he had a patent in 1798. 1 The second was more 

 simple; it was secured by a patent in 1805. It consists of four vanes revolving 

 in a cylinder round its axis. The vanes are like those of a smoke jack, but of 

 thickness sufficient to form a groove in their edges, to hold stuffing for the purpose 

 of making them steam-tight in their action. They are mounted on an arbor, which 

 has a hollow nave in the middle. Into this nave the tails of the vanes are inserted, 

 and each opposite vane affected alike by having a firm connexion with one 

 another ; so that if the angle of one of the vanes with the arbor be altered, the 

 opposite one will be altered also, and the opposite ones are set at right angles to 

 each other ; so that when a vane is flatly opposed to the steam, the opposite vane 

 will present its edge to it, and thus they are constantly doing in their rotation on 

 their common arbor ; so that the steam acts against the vane on its face for about 

 a quarter of a circle, or ninety degrees, in the cylinder where it is destined to act ; 

 and as soon as it has gone through the quarter of the circle, it instantly turns its 

 edge to the steam, while at the same instant another vane has entered the working 

 part of the revolution, and the rotation proceeds without interruption. This engine 

 was to be furnished with the condenser and discharging pump of Watt, but Horn- 

 blower added what he considered an improved method of discharging the air from 

 the condenser. 



It is easy to show that the friction, and other sources of loss of power, are much 

 greater in the rotary than in the rectilineal action of steam, while the loss by 

 rendering a reciprocating motion rotary is very small (Sect. iv. and vn.) ; but I 

 notice this as one of the most simple combinations proposed for a rotary engine. 



33. A series of experiments on the elastic force of steam from 32 to 212 was 

 published by Mr. Achard in 1782. He also examined the elastic force of the 

 vapour of alcohol ; and observed, that when steam and alcohol vapour were of 



1 Repertory of Arts, vol. ix. p. 289. Old Series. 



