Back in 1772, James Watt wanted to compare the power of horse driven machinery with steam machinery.

Over a four hour period, he observed that a “brewery horse” could lift 180 lbs

by turning and coiling rope around a 12 ft radius mill wheel, 144 times per hour.

Multiplying up, that is 32,572 ft lbs per minute.

Presumably coal, or water, was being raised out of a mine shaft.

A year later, 1773, Messrs Watt and Boulton agreed that the horsepower shoud be 33,000 ft lbs per minute.

(Easier to calculate?)

So, the weight was lifted a vertical distance of 2 x pi x Radius x No of revolutions

and the rate at which Ft lbs of work was done is: Weight in lbs x 2 x pi x 12 ft x no of revolutions / minutes taken But to convert to HP - divide by 33,000 HP = Lbs weight x 2 pi Foot Radius x RPM / 33,000It would be nice if our Stirling engines could make horsepower, but we are not there yet.

There are 746 watts to a horsepower – so it is more convenient to rate our engines in watts.

Converting to kilograms and centimetres, the formula becomes:-

Rather than raising weights in a mineshaft, Gaspard de Prony devised his Prony Brake.Watts = WEIGHT Kg x DIAMETER cms x RPM / 195

Courtesy of action, and its equal and opposite reaction,

a brake clamped to an engine shaft with a lever, will exert force at the end of that lever.

And the force, or weight, can be measured with a scale.

But, if you have yet to catch up with “metric matters”, the Imperial formula is

(It should be divide by 1,008,405. But I'll divide by 1 million) .......BACKWatts = WEIGHT ounces x RADIUS inches x RPM x 746 / 1 million

These re-enactments are from a 1937 Chevrolet film.

BY HORSE:---

BY STEAM:---

Is it actually the man who is pumping the water?

THE PRONY BRAKE