In 1895 Edward J. Willis of Richmond, Va., modified the design of the planimeter he patented in 1894, adding the frame found on 1994.0356.01, MA.324247, and MA.323703. He also devised an attachment that would allow users to directly read the horsepower produced by a steam engine, instead of having to calculate it from the mean effective pressure given by his planimeter. Between 1899 and 1905, James L. Robertson & Sons started to sell the attachment and a set of tables for making readings, charging $20.00 in addition to the $18.00 price for the Improved Willis Planimeter.
Charles F. Engman, who owned this horsepower attachment, purchased his planimeter (MA.323703) before 1901. It is likely he bought the attachment at the same time. The German silver instrument has two rectangular scales, held together at one end with a thumbscrew. Each of the scales is 13" long. On one side, they are divided into 200 units, for the 200 maximum revolutions per minute of slow-speed engines. On the other, they are divided into 750 units, for the maximum 750 rpm of high-speed engines. One of the scales is marked at one end on the high-speed side with a serial number: 111.
Two slides with adjustable thumbscrews are placed on the scales. Points on the slides grip the paper. Corks are placed over the points to protect the case when the instrument is stored. A tracer arm is 13" long and may replace the tracer arm on a Willis planimeter. The planimeter is then used on top of the attachment, with the measuring wheel on one scale and the tracer point on the other.
A wooden case covered with black leather is lined with purple satin and velvet. The inside of the lid is marked: HORSE POWER ATTACHMENT (/) FOR THE (/) Improved Willis Planimeter. (/) PATENTED JULY 9th, 1895. (/) Manufactured by JAMES L. ROBERTSON & SONS, (/) NEW YORK, U. S. A.
The object was received at the museum in 1964.
References: Edward J. Willis, "Planimeter" (U.S. Patent 529,008 issued November 13, 1894; reissued as 11,568 September 22, 1896) and "Planimeter" (U.S. Patent 542,511 issued July 9, 1895); catalog of James L. Robertson & Sons (New York, ), 29–31; James L. Robertson & Sons, Steam-Engine Indicators and Their Attachments (New York, 1905), 23; Hawkins' Indicator Catechism (New York: Theo. Audel & Co., 1903), 135–139; Hyman A. Schwartz, "The Willis Planimeter," Rittenhouse 7, no. 2 (1993): 60–64.