This beautifully restored Minneapolis is pulling on a Prony brake which is used for measuring the horsepower output of an engine.
Nowadays, engines are hooked up to a dynamometer to measure the power output, but it hasn't been very many years since engines were hooked up with a flat belt instead of a power-takeoff shaft.
The operator of the brake is adjusting the tension on the brake band to find the spot where the engine cannot maintain the running RPM. At that point, calculations are made based on the pressure exerted on the scale, and the RPM of the engine.
But; no pencil pushing here to crunch the numbers. The assistant is punching figures into a pocket calculator to give instant gratification to the engineer. Oh well; pocket calculators have been around forty years now, so they are approaching antique status, too.
From
ehow.com:
"you calculate the horsepower of the engine by measuring the braking force in pounds being applied at the end of the brake arm at the engine's stall point, multiplying that number by the circumference of the circle the end of the brake arm would describe if it were free to rotate, and by the RPM of the engine at the stall point. Then you divide that product by 33,000 to get the brake horsepower rating for that engine. The brake horsepower rating is equal to the maximum power the engine can put out."
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