Steam- Engines in Cornwall. 43 



very accurately ; c is the bar by which it is lifted. At dd it is 

 packed with the usual materials, e e is a ring by which the 

 packing is kept in its proper situation ; // is a second ring 

 resting on e e, and is kept in its place by the screws g g, by 

 means' of which also the packing is kept in a proper state of 

 compression. It is evident that the steam can exert no pres- 

 sure to prevent its being lifted ; nor when it is closed has the 

 vapour within any power to open it. It so completely answers 

 the purpose intended, that an infant might Hft the valves of a 

 90 inch engine. The packing is not a very desirable concomi- 

 tant, and it also increases the dimensions of the valve. 



In the Philosophical Transactions for 1827, Mr Davies Gil- 

 bert, the illustrious President of the Royal Society, has pub- 

 lished some interesting observations on the steam-engine. But 

 in estimating the efficiency (/ X s) or force, multiplied into the 

 space through which it acts, he assumes both these functions 

 to be invariable. Now, in the present case, the value ofy is de- 

 pendent on the quantity of water evaporated by a given por- 

 tion of fuel. The writer of this has already shown that this is 

 in different engines very variable. The value of s must ([fric- 

 tion disregarded) depend on that of f. But if friction be in 

 operation, and has different amounts in various engines, we can- 

 not compare their efficiency until we reduce the value of fric- 

 tion to a determinate standard. But if the value of fhe like 

 in two instances, that of *, with the requisite correction for 

 friction, may be determined by the duty performed. And as 

 the ratio which s will bear to/ can be determined only by ex- 

 periment, it does not seem that we have any means of introdu- 

 cing the function f into the estimate of efficiency, without ma- 

 king friction another element ; consequently that duty and ef- 

 ficiency are identical, except when expansive working obtains, 

 and then the value of the advantage thus gained is the mea- 

 sure of their difference. Hence the duty is sometimes greater 

 than the efficiency, but never less. In calculating the duty of 

 a steam-engine, it is to be feared we cannot arrive at any very 

 accurate result, in the case of its being applied to spinning-ma- 

 chines, mills, &c. as our knowledge of the resistance opposed by 

 such apparatus is very limited. The only means of arriving at 

 any tolerable approximation appears to us to be, by ascertaining 



