It is quite possible, that the best living author of popular science is Sam Keen. His book The Disappearing Spoon is on par, or perhaps, in my weaker moments, slightly better than Isaac Asimov’s Building Blocks of The Universe. I never thought that would happen. His book The Violinist’s Thumb was very engaging, after a somewhat slow beginning. I did not have high expectations for his new book, Caesar’s Last Breath, as I have read extensively about gasses and the atmosphere; I expected much to be a repeat. While some was, the majority of the book was unfamiliar, offered rich details concerning what I did know, and is quite interesting. His asides are as engaging as his intended narrative. When Keen arrived at James Watt (1736-1819), he offered a perspective that had passed me by:
The expansion into new markets got Watt thinking about steam engines in more grandiose terms as well. To most people, the engines were just tools built to accomplish a specific task—pump water, drive a lathe, whatever. Watt envisioned the engines more as universal sources of energy—machines capable of powering any mechanical process. As an (anachronistic) analogy, most people saw steam engines as something like calculators: proficient at one task but useless beyond that. Watt dreamed of building the steam equivalent of computers, machines versatile enough to work in any industry.
Rather than calculate every factory’s case separately, Watt invented a universal standard of comparison, the horse power. He defined this rather literally by watching several horses push a mill wheel around and then calculating how far they moved the weight in a certain amount of time (550 foot-pounds per second, he found). This unit was shrewd in several ways. By invoking horses, Watt slyly reminded factory owners what they could give up—all the oats and broken legs and
vet bills. Customers also understood the unit intuitively. If ten horses had run their mill wheel before, well, they needed a ten-horsepower engine.
Scientifically, the idea proved prescient as well. Over the next century chemistry and physics would be dominated by thermodynamics, the study of heat and energy. Energy is a vast topic in science, popping up in all sorts of different contexts, and scientists needed a standard unit of comparison to understand how quickly different processes absorbed and released energy. The horsepower fit the bill perfectly. Little did those scientists know that the whole idea started as a marketing scheme by James Watt.
(As thermodynamics branched out into new phenomena, however, like light and magnetic fields, the absurdity of the name “horsepower” became obvious—as if you could still hitch old Bessie to the apparatus. In 1882 physicists finally voted to establish a new universal unit of power, which applies just as readily to light bulbs and refrigerators as to engines for raising water by fire. They called it the watt.) pp 173-174
The phrase horsepower is such a powerful meme that if you look up the specifications for a 2017 Dodge Charger, its power output is given only in horsepower as:
SAE Net Horsepower @ RPM : 485 @ 6100
The marketing power of horsepower caused misguided people to define a metric horsepower. A mechanical horsepower is about 745.7 watts, whereas a metric horsepower is approximately 735.5 watts. Electric motors in Europe have both metric and mechanical horsepower ratings. There is also a boiler horsepower, used to rate steam boilers, which is equal to 9809.5 watts.
The output of a horse is not constant, there is a peak and sustained value. Data gathered at the 1926 Iowa State Fair indicates that over a few seconds a horse can achieve a power output of 14.9 horsepower (11.1 Kilowatts). The data indicated that indeed, the horses measured achieved about a 1 horsepower sustained output.
The continued use of horsepower throughout the world demonstrates the power of romantic metaphor over a carefully designed quantity, even when the resulting unit is named after the horse-trading marketer who came up with horsepower.
The Metric Maven has published a book titled The Dimensions of The Cosmos. It examines the basic quantities of the world from yocto to Yotta with a mixture of scientific anecdotes and may be purchased here.