Figure 24. — Thomas C. Mendenhall (1 841 -1924). 

 Although largely self-educated, he became the first 

 professor of physics and mechanics at the Ohio 

 Agricultural and Mechanical College (later Ohio 

 State University), and was subsequently connected 

 with several other universities. In 1878, while 

 teaching at the Tokyo Imperial University in 

 Japan, he made gravity measurements between 

 Tokyo and Fujiyama from which he calculated 

 the mean density of the earth. While superintend- 

 ent of the U.S. Coast and Geodetic Surve) . 

 1889-94, he developed the pendulum apparatus 

 which bears his name. 



length, and ^-second time of swing. The pendulum 

 was hung by a single knife edge, which rested on a 

 plate that was supported by a tripod. The pendulum 

 was swung in a chamber from which air was exhausted 

 and which could be maintained at any desired 

 temperature. Times of swing were determined by the 

 observation of coincidences of the pendulum with 

 chronometer signals. In the final form a small 

 mirror was attached to the knife edge perpendicular 

 to the plane of vibration of the pendulum and a second 

 fixed mirror was placed close to it so that the two 

 mirrors were parallel when the pendulum was at rest. 

 The chronometer signals worked a relay that gave a 

 horizontal spark which was reflected into the telescope 

 from the mirrors. When the pendulum was at rest, 

 tin image of the spark in both mirrors appeared on 



the horizontal cross wire in the telescope, and during 

 oscillation of the pendulum the two images appeared 

 in that position upon coincidence. In view of the 

 reduced size of the pendulum, the chamber in which 

 it was swung was readily portable, and with an 

 improved method of observing coincidences, relative 

 determinations of gravity could be made with rapidity 

 and accuracy. 



By 1887 Maj. von Sterneck had perfected his 

 apparatus, and it was widely adopted in Europe for 

 relative determinations of gravity. He used his 

 apparatus in extensive gravity surveys and also applied 

 it in the silver mines in Saxony and Bohemia, by the 

 previously described methods of Airy, for investiga- 

 tions into the internal constitution of the earth. 



On July 1, 1889, Thomas Corwin Mendenhall 

 became superintendent of the U.S. Coast and Geo- 

 detic Survey. Earlier, he had been professor of 

 physics at the University of Tokyo and had directed 

 observations of pendulums for the determination of 

 gravity on Fujiyama and at Tokyo. Supt. Menden- 

 hall, with the cooperation of members of his staff in 

 Washington, designed a new pendulum apparatus 

 of the von Sterneck type, and in October 1890 he 

 ordered construction of the first model/ 3 



Like the Von Sterneck apparatus, the Mendenhall 

 pendulum apparatus employed a nonreversible, in- 

 variable pendulum %-meter in length and of slightly 

 more than }i second in time of swing. Initially, the 

 knife edge was placed in the head of the pendulum 

 and hung on a fixed plane support, but after some 

 experimentation Mendenhall attached the plane 

 surface to the pendulum and hung it on a fixed knife 

 edge. An apparatus was provided with a set of 

 three pendulums, so that if discrepancies appeared 

 in the results, the pendulum at fault could be de- 

 tected. There was also a dummy pendulum which 

 carried a thermometer. A pendulum was swung 

 in a receiver in which the pressure and temperature 

 of the air were controlled. The time of swing was 

 measured by coincidences with the beat of a chronom- 

 eter. The coincidences were determined by an 

 optical method with the aid of a flash apparatus. 



The flash apparatus was contained in a light metal 

 box which supported an observing telescope and 

 which was mounted on a stand. Within the box was 



" T. C. Mendenhall, "Determinations of Gravity with 

 the New Half-Second Pendulum . . . ," Report of the Super- 

 intendent of the U.S. Coast and Geodetic Survey for 1890-91 (Washing- 

 ton, 1892), part 2, pp. 503-564. 



332 



BULLETIN 240: CONTRIBUTIONS FROM THE MUSEUM OF HISTORY AND TECHNOLOGY 



