Steam- Engines in Cornwall. 39 



air into the fire-place ; but if the fire do not burn briskly, there 

 is a probability that a portion of coal gas will pass off without 

 undergoing combustion, and consequently without affording 

 any assistance. It is perhaps almost unnecessary to observe, 

 that no more air should be admitted into the fire-place than 

 is requisite for the maintenance of the proper degree of activi- 

 ty in the combustion and draught, and this degree must be the 

 smallest of which the demands of the engine will admit. It is 

 an object of importance, that the pipe by which the steam is 

 conveyed from the boiler to the cylinder should be consider- 

 ably inclined towards the former ; thereby permitting the re- 

 turn to the boiler of any water which may have obtained from 

 condensation in the pipe. For were this water to enter the 

 cylinder, it might be easily apprehended that its effects would 

 be very detrimental ; it would probably occasion further con- 

 densation, and very much augment the adhesion of the pack- 

 ing of the piston to the cylinder. Hence the importance of 

 coating the steam-pipe with a considerable depth of non-con- 

 ducting matter. This point is much insisted on by Mr Grose, 

 who maintains that the adhesion, even when the packing is 

 well oiled, is much greater at low than at high temperatures. 

 It seems that a load of between nine and twelve pounds on 

 the inch of the area of the piston, is the most advantageous 

 to the performance of the steam-engine, and we think Mr 

 Watt entertained an opinion not very different from this, al- 

 though we are not prepared to assign any very satisfactory rea- 

 son for its being so. The whole of the pumping engines in 

 Cornwall raise the column of water during the returning stroke, 

 and, as but few of them work without an interval between each 

 stroke, the means of considerably assisting their operation is 

 thus afforded. A counterpoise to the weight of the pump 

 rods is alvvays required, and the quantity of this is so adjust- 

 ed as to occasion the return to be made very slow, and to ter- 

 minate but an instant before it is necessary to make the suc- 

 ceeding stroke. Hence it is evident, the more slowly the re- 

 turning stroke is made, the smaller the quantity of steam re- 

 quisite to make the working stroke. But it is obvious that 

 this assistance can never be given either to rotatory or to 

 double reciprocating steam-engines, that which would have 

 been gained on one hand being lost on the other ; consequently, 



