360 
Steam Engine. 
ly applied) though it is much less certain and accurate 
than other methods well known to mechanical men. The 
imcertaiiity of calculating by horse-powers arises from 
various causes ; such as the great differences of ability 
between the strong and heavy horses used in London^ 
and those of not half the strength used in various parts of 
the country the greater or less degree of speed during 
work ; the quantity of re-action against which they are 
urged to pull ; the shorter or longer time of work ; their 
food, stabling, &c. 8cc. And this uncertainty, as may 
easily be conceived, is so great, that the words horse- 
power cannot practically be applied, otherwise than to de- 
note a certain quantity of mechanic effect agreed upon and 
understood between engineers, and must not be under- 
stood to denote any elementary measure, capable of being 
worked out or inferred within any reasonable or useful 
limits, from the real power of the horse himself. 
It therefore follows of necessity, that the engine must 
be examined by first stating its mechanic effect ; that is 
to say, how many pounds weight it is capable of raising 
tiirough a given space in a given time, that is to say, 
through the height of one foot during one minute, and 
then dividing this sum by the like effect producible by 
one horse, according to the statements and practice of en- 
gineers of the first reputation. I confine myself to Messrs. 
Boulton and Watt, Dr. Desaguliers and Mr, Smeaton. 
The practice of Messrs. Boulton and W att is, to con- 
sider a horse as capable of raising a certain weight, which 
is stated to be of 3:.', 000 pounds avoirdupois, one foot 
high in one minute. Desaguliers^ results brought to the 
same form, give 27,500 pounds ; and Smeaton^s 22,916 
pounds, under the same circumstances. The lowest of these 
performances are more than equal to the average power 
of a horse employed in husbandry for eight hours per day^ 
If the dianieter of the c^dinder be multiplied by itself 
