Hart's Mercantile Computing Machine

The William Hart story had many twists and turns. He was born in the state of New York in 1829. He moved to Kirksville, Missouri from Maysville, Wisconsin in 1867 and opened a jewelry store, doing business as W. Hart Jeweler, but later sold his jewelry business. Because of declining health, Hart said that he wanted to “take up something giving more outdoor exercise.” The 1880 Federal Census shows Hart’s occupation as a watchmaker; whether he was actually pursuing a career at that time is not known. In 1886 Hart went back to jewelry business business as a owner of Hart’s Jewelry Store. In 1890 a fire destroyed the home and store of Hart, his son Volney died, William Hart barely survived. In 1892, Hart was back in the jewelry business again, known now as Hart & Son. After the death of his wife in 1894 Hart leaved the Kirksville and probably USA. William Hart died on April 2, 1907, in Las Mochis, Sinaloa, Mexico.

Hart had two patents for clock escapements in the early 1860s, but these escapements were never used. That's not the case however with his calculator, patent No 199289 from January, 1878 (see the lower drawing).

The patent drawing of Hart's calculator

The device was sold under the name Hart's Mercantile Computing Machine. There are several of these calculators known to exist now. One of Hart's calculators is on display in the Smithsonian National Museum of American History. There is an uncertainty as how many were actually produced. Hart may have hand made a few. Smithsonian’s calculator indicate Scovill Manufacturing of Waterbury, Connecticut, and a company by the name of J. W. Strange, made it. Scovill was a maker of brass products. The cylindrical case for the calculator is made of wood (see the lower photo).

Hart's Mercantile Computing Machine

The diameter of the instrument is 12.4 cm. It has two discs numbered from 0 to 99. There is a long hand with a pointer coming down from its end which rotates the upper disc over the lower one to add numbers up to 99. When the total exceeds 99, a hand like the short hand of a watch automatically advances one to indicate the hundreds value (the adder has a single carry). Sums of up to 9999 can be indicated.