E.D. Preston, of the United States Coast and Geodetic Survey, made many astronomical and geophysical observations between 1883 and 1899, particularly on the Hawaian Islands. His first paper was a "Report of the Eclipse Expedition to Caroline Island, May 1883" in the Memoirs of the National Academy of Sciences (1884). However, most of his results were published in the annual Report of the US Coast and Geodetic Survey. His reports dealt with the following topics: Determination of latitude and gravity for the Hawaian government (1888); star observations (1888); observations of the transit of Mercury in May 1891 at Waikiki, Hawai (1891); gravity and magnetic observations on Hawai and Honolulu (1893); the latitude, gravity and magnetic elements observed by him in Hawai (1894); determination of the constant of aberration from latitude observations with the zenith telescope on Honolulu and at San Francisco (1895); a method for determining star positions (1895); the instruments of the United States Naval Observatory (1895/6); and geodesy in the United States (1899).
From October 1889 to May 1890 Preston was a member of the United States Eclipse Expedition to West Africa in USS Pensacola. The expedition was led by Professor D.P. Todd, while C. Abbe* was its meteorologist. Preston was in charge of gravity and magnetic observations. He made magnetic observations at 14 stations, five on the west coast of Africa and eight on islands in the north and south Atlantic, while his gravity observations extended from Cape Town to Washington.
Arriving at the Cape on 18 January 1890 the expedition was received by Dr David Gill* of the Royal Observatory, Cape of Good Hope. Preston, assisted by Cadet Patton of the United States Navy, made a series of pendulum observations to measure the local strength of the earth's gravity, and measured the magnetic declination and dip, as well as the horizontal and total strength of the earth's magnetic field.
After the expedition's return Preston wrote a paper on "The study of the earth's figure by means of the pendulum" for the American Journal of Science (1891) and reduced his observations. The results were published in a report, "On the secular variation and annual change of the magnetic force at stations on the west coast of Africa and at some islands in the North and South Atlantic". The report, compiled and discussed by Charles A. Schott, was included as Appendix 3 in the Report of the US Coast and Geodetic Survey for 1891, while the results were also published in the institution's Bulletin No. 23.