MADE IN THE EFFICIENCY OF STEAM ENGINES IN CORNWALL. 123 
and September last, work the said pumps at the rate of 6 strokes of 5^ feet 
long each in every minute, which amounts to 8640 strokes per 24 hours. 
“We have also made an accurate calculation, by which it appears that when 
the new fire-engine to be erected by Messrs. Boulton and Watt is completed, 
and actually works a pump of the same depth of 58 fathoms and 17 inches in 
diameter at the rate of 6 strokes of 5^ feet long each in a minute, and conse- 
quently making 8640 strokes per 24 hours, it will draw a quantity of water 
equal to that now drawn by both the present engines, and consequently what- 
ever smaller quantity of coals it uses than 14’080 bushels for 61 days when 
going at the rate of 6 strokes per minute, will be the real savings in fuel oc- 
casioned by the said new engine at that rate of going. 
(Signed) “ James Watt. 
“ Matthew Boulton. 
“ H. Hawkins Tremayne, 
“ Richard Williams. 
“ John Williams. 
“ Thomas Brown.” 
A pound avoirdupoise lifted through one foot had not at that period been 
established as the dynamic unit. 
The product of pounds raised, and of the number of feet through which they 
are lifted in a given time, divided by the number of bushels of coal (supposed 
to weigh 84 pounds) burnt in the same interval, give what is now termed the 
duty of the engine, and afford a perfect criterion of its comparative merit. 
The most convenient method of forming this estimate is by multiplying 
together, the diameter in inches squared, of the lifting box, or of the plunger 
piston, the height of the lifts in fathoms, and 2.04 (log. 3.3095101) the weight 
in pounds avoirdupoise of a cylinder of water one inch in diameter and six 
feet long, the product gives the weight. 
This, multiplied by the length of the stroke in feet, and by the number of 
strokes in a given time, and finally divided by the number of bushels of coal 
consumed, will give the duty. 
