A brilliant marketing idea

When Boulton and Watt first began to introduce their rotative steam engines into manufactories, about 1784, they had to commit to a certain power their machine would deliver. Boulton & Watt wanted to express this power in such terms as would be readily understood by the persons who were likely to want such engines. The machinery in the great breweries and distilleries in London was then moved by the strength of horses, and the proprietors of those establishments, who were the first to require Mr. Watt's engines, always inquired what number of horses an intended engine would be equal to.

In consequence, Mr. Watt made some experiments on the strong horses employed by the brewers in London, and found that a horse of that kind, walking at the rate of 2½ miles per hour, could draw 150 pounds avoirdupois, by means of a rope passing over a pulley. A least 5 different weights for a pound existed at the time in England. According to Watt's calculation 33 000 pounds were equal to 528 cubic foot of water, which makes 1 avoirdupois pound equal to 0.4531kg. Today, 1 avoirdupois in the metric system is assumed to be approximately 0.4536 kg, an error of aprr. 0.1%, which can be neglected. So Watt dominated one horse power as to raise up the weight of 150 avoirdupois pound with vertical motion, at the rate of 220 feet per minute to serve for a measure of the power exerted by his steam-engines. As the steam engines in the beginning were mainly used to raise water from mines the horse power is often expressed as the power raising 33 000 pounds of water one foot high in a minute. [1].

According to a legend the brewer in London, a canny Scott, chose his strongest horse an was driving it eight hours to the limit of its endurance [2]. Smeaton and other engineers made many observations on the work actually performed by horses when working regularly in mills, and the results seem to show that 22 000 lbs., raised at the rate of 1 foot per minute, may be taken for a real horse-power, or as the exertion that a good horse can overcome with so much ease as to continue to work for 8 hours per day. [3]

In Germany in a similar experiment a horse raised 75kg in 1 second to a height of 1m.  Probably the real figure was rounded to 75kg as was the the 33,000 pounds of water. This is the reason why one metric horse power as 75kgm/s is slightly bigger than 1 imperial horsepower of 76.04kgm/s. In SI units the metric horsepower is 735.5 Watt and the imperial horsepower is  745.7 Watt. In this case study I will use the imperial horse power as the historical documents mostly use the imperial horse power and thus the historical results are easy to compare. Still most of us arr used to horse powers from the car engines, although with electrical cars this may change.