Mr. Henwood on the Cornish Pumping-Engines. 485 



2,802,247 cubic feet (of a pressure of 10*2 lbs. on the inch) 

 would have been requisite; whereas but 2,153,647 cubic feet 

 only could be obtained from the quantity of water evaporated. 

 Consequently but the 0"768th of the contents of the cylinder, 

 &c., could, on an average, have been filled with steam of that 

 force; and the remaining 0'232 of the stroke must therefore 

 have been performed by virtue of the momentum acquired by 

 the machine in the early part of the working stroke. 



This 0*232 part of the whole is therefore the benefit ob- 

 tained by working the steam expansively ; although the result 

 obtained by the indicator exhibits a still greater (about 0*388) 

 advantage. The cause of this difference it is notvery easy to 

 assign satisfactorily. It is just possible that it may be from 

 the fluctuating pressure of the steam (from 77"25 to 47*22 lbs. 

 on the inch) during the experiment, giving a result differing 

 on a mean more than 61*8 lbs. on the inch, (the force when 

 the curve represented in the figure p. 482 was obtained,) does 

 from the average elasticity during the observation (tj4"l lbs.). 

 But perhaps it may more probably be from the steam, even 

 when expanded to a less force than 10'2 lbs. on the inch, still ex- 

 ercising a beneficial influence in assistance of the momentum by 

 which the latter part of the working sti'oke is performed. 



In a first attempt at such a comparison, which I believe is 

 here made, it may perhaps excite no great surprise that there 

 is not a more exact coincidence between the results obtained 

 by these very different modes of inquiry. 



II. The dull) jjerformed xoith a given quantity of fuel. — The 

 experiments with an object to determining the duty performed 

 with a known quantity of fuel, were made on Wilson's engine 

 at Huel Towan ; on Swan's engine at Binner Downs Mine ; 

 and on Hudson's engine at East Crinnis Mine*. These 

 were among the best engines in Cornwall, and they were se- 

 lected on account of the very varied circumstances under which 

 they worked. 



At Huel Towan the cylinder with its cover and bottom 

 were surrounded with a case or jacket, filled with dense steam 

 from the boiler; and these, with the steani'pipes, nozles, &c., 

 were covered with saw-dust from 16 to 20 inches deep. The 

 boilers had a layer of ashes, of about the same thickness, 

 ])laced on them. 



There was no steam-case at Binner Downs, but there were 

 small fires on each side of the cylinder, and the flues from 

 them were carried spirally round it; another little fire was 

 placed beneath the steam- nozle, from the boiler, and its flue 



* Tlie <;ni,'ineers were respectively, Mr. Grose, Messrs. Gregor and 

 Thomas, and Mr, Sims. 



