200 MR W. J. M. RANKINE ON THE POWER AND ECONOMY 
Comparison of the Theory with Mr WicKsTEED’s Leperiments on the Cornish Pumping 
Engine at Old Ford. 










Precere anil! Steam cit Maximum | Lb. of Steam expended Duty of one Ib, of 
Number the Boiler. v Ratio of Pressure in per stroke, \ Steam, J 
of meee ie aoe > | off at a of erp ee hae d =e Difference. - Sie oF ee Difference. 
earl HEAD DE TING ; square neh aheesy. Saat Theny. SNERE 
fl a foot-Ibs. | foot-Ibs, 
B. 30°45 | 0°603 | 1:605 | 14:27 | 7-781 | 7-586 |—0-:245} 71530 | 73860 
C. | 33:20 | 0-477} 1:988 | 15:59 | 6:963 | 6-463 |—0:500! 79936 | 86123 
D. 39-2 0:397 | 2:342 | 16-9 6-236 | 6:200 |—0:086| 89275 | 89776 
E. 41-2 0°352 | 2-605 | 17-89 | 5:905 | 5985 |+0:085| 94258 | 93002 
F. 45-7 0313 | 2-882 | 18:98 | 5-626 | 5-470 |—0:156| 98940 | 101756 
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) 











This comparison sufficiently proves that the results of the theory are practically 
correct. 
It is remarkable, that in every instance except one (experiment E) the expe- 
rimental results shew a somewhat less expenditure of steam per stroke, and a 
greater duty per pound of steam, than theory indicates. ‘This is to be ascribed to 
the fact, that although the action of the steam is computed theoretically, on the 
assumption that during the expansion it is cut off from external sources of heat, 
yet it is not exactly so in practice; for the cylinder is surrounded with a jacket 
or casing communicating with the boiler, in which the temperature is much 
higher than the highest temperature in the cylinder, the pressure in the boiler 
being more than double the maximum pressure of the steam when working, as 
columns (2) and (5) shew. There is, therefore, a portion of steam, of whose 
amount no computation can be made, which circulates between the boiler and the 
jacket, serving to convey heat to the cylinder, and thus augment by a small quan- 
tity the action of the steam expended; and hence the formule almost always 
err on the safe side. 
Supposing one pound of the best Welsh coals to be capable (as found by Mr 
WicksTEED) of evaporating 9°493 lb. of water at the pressure in the boiler during 
the experiment F, then the duty of a Cornish bushel, or 94 lb. of such coals, in 
the circumstances of that experiment would be— 
By theory, : P e 88,288,000 ft. Ib. 
By experiment, : c 90,801,000 «-- 


Difference, . + 2,513,000 --- 




