Rotatory Steam ■Engine. 43 



shall give out its force in setting the medium in motion, and' 

 in overcoming the very great resistance of the liquid in all the 

 pipes, passages, and valves through which it is transmitted to 

 alternate sides of the wheel in every revolution : the whole of 

 this force is subtracted from useful effect, and becomes power 

 lost. 



In those which move weights from and towards the circum- 

 ference, there are mere groups of reciprocating pistons without 

 cranks, and share the evils to be explained in Class V. ; in fact, 

 in the engines of Watt and Witty of this class, we have a series 

 of reciprocating engines ranged round a wheel to do the work 

 of one. 



In the case of the fluid medium, we have not only a loss of 

 all the power expended in nioving the medium itself, but also 

 the additional loss of effect encountered in all modes hitherto 

 adopted for applying a fluid to the rotation of a wheel, a loss, 

 in the best examples ever presented, amounting of itself to 

 more than one-sixth part of the power. 



Class III. — The Engine of Hydrostatical Beaction is more 

 effective than either of the former classes. As invented by Watt 

 in 1769, it consisted of steam- vessels in the form of hollow rings 

 or circular channels, with proper inlets and outlets for the 

 steam, mounted on horizontal axles, like the wheels and buckets 

 of a water-mill, and wholly immersed in some fluid. This 

 wheel was made of iron, six feet in diameter, and the reaction 

 of mercury was employed to give revolution to it ; the engine 

 moved, but was found to be inefficient and abandoned, al- 

 though it had been tried in very favourable circumstances. 



The principle of action is this : Steam is admitted into a cir- 

 cular channel or chamber on the circumference of a wheel ; this 

 chamber is partially filled with some liquid, the pressure of the 

 steam is expended in pushing the mercury in one direction, 

 and the end of the chamber in the opposite way, so that while 

 the liquid is thus forced out of the chamber, the chamber is by. 

 an equal force pushed away from the liquid ; the wheel is thus 

 turned round. 



It is apparent that a part of the force is employed in pro- 

 pelling the wheel, and the remainder is expended in overcom- 

 ing the resistance of the liquid of reaction, and expelling it 



