Processes in the Useful Arts. 163 



As air and steam- out of contact with its generating water, follow the 

 same ratio of expansion, — in the absence of experiments on the subject, 

 it seems reasonable to suppose that their capacities vary as one another, 

 and consequently that the capacity of the steam in the case — 1,910, whilst 

 that of the steam contained in the cylinder at the termination of the work- 

 ing stroke — .8983 for equal volumes ; and for equal weights that of the 

 former, — .02684 whilst that of the latter — .03607. It consequently re- 

 quires that 3607 parts by weight of steam of a pressure equal to that of 

 71,15 inches of mercury be cooled; in order to heat 2684 parts of the 

 pressure of 24,9 inches, the same interval of temperature. 



From the law of Marriotte it follows that the elastic force of a given 

 weight of any elastic fluid X the sphere of its action, will give the same 

 quantity, whatever may be the temperature, pressure, or density ; pro- 

 vided the whole quantity of heat in the mass suffer no increase or diminu- 

 tion ; and hence friction and capacity disregarded, we obtain the same re- 

 sult, whether a given quantity of water and caloric, in the form of vapour, 

 act on a surface of 8, or 80 inches in diameter. 



In the use of a steam case it appears that a portion of steam is destroy- 

 ed, in order to communicate to another portion a force so considerably less 

 than what would have been obtained by adding their volumes together, as 

 the difference between 2684 and 3607. The former representing the effect 

 produced on the steam in the cylinder by the abstraction of heat from the 

 case ; and the latter what would have been the effect of the steam conden- 

 sed in the case, had it been employed in the cylinder. 



But the quantity of injection water will be augmented by the steam 

 within the cylinder acquiring an increased quantity of heat after its admis- 

 sion there ; the load of the air pump being of course proportionally great- 

 er ; the surface exposed to the cooling influence being enlarged by the 

 application of a case. Mr Woolf and Mr Grose usually employ steam 

 cases, which, with the upper parts of the boilers, the steam pipes, cylinder 

 cover, nozzles, &c. are surrounded and covered with from 10 to 14 inches 



thickness, of ashes, saw dust, or powdered charcoal. 



iefore the application of which I was informed by Mr Hocking, en- 



ieer at the Consolidated Mines, the quantity of steam condensed in the 

 case of a 90 inch cylinder, and working with steam of a pressure of about 

 inches of mercury gave 81,6 cubic feet of water in 24 hours. 



Mr Sims has recently adopted a plan, which, if it can be so carefully 

 managed by the engine-men as to avoid charring the packing of the piston, 

 seems preferable to a steam case. It is making the flue, from the boiler 

 to the chimney, around the cylinder. But it is doubtful whether the ad- 

 vantage obtained in the cylinder be not counterbalanced by the increased 

 burden on the air pump. 



4. On the quantity of heat which passes to the chimney, in Engines work' 

 ing with high pressure steam. 

 The subject of the economy of high pressure steam is one of the most 





