1046
THEORY AM) PRACTICE OK ENGINEERING.
IW II.
medium effect of a horse will raise each minute 22,000 pounds a foot high, although horsepower is defined to be competent to raise 33,000 pounds in the same time; but when em-ployed in mills the action of the animal is perpetually restrained by the circular path inwhich he walks, and the power is not so great as when pulling in a direct line; and in some
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Fig. 1606.
H0K3E-M1LL
measure to alleviate this impediment, it is an invariable rule never to make the diameterof the horse-path less than 18 feet in diameter. Rotation is given to such mills either bylarge cogged or bevelled wheels placed below or above the revolving shaft; the horses inthe one case walking on a level with the large cogged wheel.
If a steam-engine of 10 horse nominal power taken at 33,000 lbs. were to work day andnight, it would perform as much as 45 horses in the same time; and making every allow-ance for casual and necessary stoppages, every nominal horse power in a steam-engine isequivalent to four horses when working night and day, to two horses when working twelvehours a-day, and to one and a half horses when working eight hours a day.
Mills for grinding Corn by the power of steam, were first used in 1783, on the Surreyside of Blackfriars bridge, but in consequence of their being considered a monopoly in-jurious to the public, they were destroyed by fire a few years afterwards.
Since that period, Messrs. George and John Rennie have erected at the Royal Williamvictualling yard, Plymouth , one of the most perfect mills in the kingdom for grinding corn.The building is 240 feet in length, and 70 feet in height : in the centre are two steam-engines of fbrtv-five horse power; on each side are twelve pair of stones 4 feet 3 inches in