40 Mr Kenwood's account of the 



the performance of single is better than that of double recipro- 

 cating or rotatory engines. But as the latter are never work- 

 ed ** expansively,"" this accounts for a small portion of the dif- 

 ference. An advantage of some consideration is obtained in 

 the pumping engines, by allowing the exhausting valve to be 

 opened before the steam is admitted on the piston, which con- 

 sequently meets a considerably smaller resistance at the com- 

 mencement of its motion than it would have, had both valves 

 been opened at the same instant. 



It is not unusual to force the water intended to replace the 

 evaporation from the boilers into a separate vessel kept con- 

 stantly full of liquid, and around which the flue from the boiler 

 to the chimney is passed. It thus attains a temperature but 

 little below that of the water in the boilers, which are sup- 

 plied by opening a communication between them and this ves- 

 sel, into which a portion of liquid is now injected; and this 

 displaces an equal bulk of the warmer liquid which passes into 

 the boiler. There are at Huel Towan engine three boilers, 

 each about thirty-six feet long ; the outer tubes are six, and 

 the inner four feet in diameter ; the area of the fire grate is 

 in each about twenty-eight or thirty feet. The writer of this 

 notice has observed that engines with boilers of smaller capa- 

 city do not perform such duty. Some of those next in good- 

 ness have a greater and others a less reservoir of steam. It ap- 

 pears that the dimensions of Huel Towan are most efficient, 

 but that a smaller quantity is preferable to a larger. We be- 

 lieve that the importance of attending to the operation of the 

 air-pump has not since Mr Watt's time been sufficiently noticed. 

 We think the following remarks will help to place the subject 

 in a proper point of view. The quantity of water should be 

 as small as possible, not so much on account of its weight, as of 

 the greater period during which the piston of the air-pump will 

 be exposed to the atmospheric pressure. On the other hand, 

 the smaller the quantity of water injected, the higher will be 

 the temperature of the hot well, and consequently the less per- 

 fect the vacuum. It is obvious that the smaller the quantity 

 obtained, by adding the difference between the impeding influ- 

 ence of the vapour of the hot well on the piston, and its acce- 

 lerating action on the air-pump, to the whole resistance expe- 



