Hart's Mercantile Computing Machine

The William M. Hart's 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 in the square, 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).

Hart Mercantile Computing Machine patent 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. According to a local newspaper, Kirksville Missouri Democrat for July 26, 1888, by then Hart had sold 3500 of these devices and "he lately ordered one thousand more." 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.

Hart Mercantile Computing Machine

Hart's Mercantile Computing Machine in its wooden case (© Smithsonian National Museum)

The calculator is housed in a cylindrical wooden case (with cover), with diameter 13.5 cm and height is 5.5 cm. The instrument itself is made by steel and brass, and has a wooden handle.

The device consists of three concentric brass discs, a brass marker, a steel stop, and a long wooden handle with a pointer coming down from its end, which rotates the upper disc over the lower one to add numbers up to 99. Each brass disc has the numbers from 0 to 99 stamped around the edge. The two inner discs both have a circle of 100 holes just outside the numbers. The inner holes are used to add the last two digits of a number by rotation. Any hundreds value in the sum carries to the second set of holes, which are used to add hundreds and thousands places. 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.

Hart Mercantile Computing Machine

Hart's Mercantile Computing Machine