Why engine power was measured in Horse Power?

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We Mechanical Students/Engineers/People discuss about new automobiles, motors etc., everyday. We compare engine power in terms of "Horse"

So ever you taught why engine power is measured in terms of horse when there are many other animals, birds etc.,

First of of all we have to thankScottish Engineer James Watt who discovered "HORSEPOWER"

Here is History

Let's go back to 18th century and see what happens there!

We all knew that those times people use horses for transportation and for everything.

James Watt at first engines were installed and working with Ponies Lifting Coal at coal mine. These engines are not manufactured by boulton and watt, but made by others according to the drawings made by watt.

Later watt started manufacturing engines on his own. Watt was looking to market his new engine that uses about 75% of less fuel than a Newcomen steam engine and other improvements.

Of course many at the time used horses, not steam engines, so it was difficult to compare without actually buying the engine to see. How it would perform for their particular usage.

So Watt once travelling to home from office and thinking about how to market his steam engine?
out of the window some people using draft horses for their mechanical work then watt got an idea that he has to compare his engine with horse. Then he immediately went to office and started experiments how much work that typical draft horse can do? It isn't known how he came up with the numbers he did. But after number of experiments he found that a typical draft horse can lift 324 pounds (1 Pound = 0.453Kg) over 100 foot in 60 seconds. Later it was rounded to 33000 foot-pound in 60 seconds.
or 33 pounds over 1000 foot in 60 seconds or 1000 pounds over 33 foot in 60 seconds or 330 pounds over 100 foot in 60 seconds.

So