Sieam Eiwine^ 99 
o 
er of horses, to state the number of the latter to which the 
new power, under given conditions, would be equivalent ; 
and it is probable that Boulton and Watt felt that such a 
mode of comparison would be more intelligible to com- 
mon apprehensions, than a more accurate and scientific 
formula. It gave the power of an engine expressed in 
numbers, of which the ordinary strength of a horse is the 
unit. This, no doubt, is not in itself very exact, the 
unit being large, and subject to some variation. Relative- 
ly to the purpose for which it was used, it was however 
sufficiently correct ; and on this, as on many similar oc- 
casions, a more minute measurement would have been less 
useful. If a historian would express the interval of time 
between two different events, he is in general satisfied with 
counting the number of years ; and it would be a useless 
affectation of accuracy to reckon up the months, days, 
hours and minutes, that must be added or taken away, in 
order to measure the aforesaid interval with mathematical 
exactness. So, also, if a man were to ride post from 
London to York, it would serve his purpose as well to 
know the distance of these cities in miles, as in feet, inches, 
and decimals of an inch. 
Boulton and Watt, however, have not left the matter hi 
a stale that can be accounted incorrect in any case, but 
have given to it all the accuracy that can be required, when, 
from the result of experiments made with the strong hor- 
ses employed by the brewers in London, they have as- 
sumed, as the standard of a horses’s power, a force able 
to raise thirty-three thousand ib. one foot high in a mi- 
nute ; and this, no doubt, was meant to include an allow- 
ance of power sufficiently ample to cover the usual varia- 
tions of the strength of horses, and of other circumstances 
that may affect the accuracy of the result. This determi- 
nation, we think, could not be unknown to Mr. Gregory, 
.md certainly not to his coadjutor. If, in forming the esti- 
