which observations could be made, in comparison with direct measures of the earth, has caused it to be regarded as a most important geodetic instrument.
As early as 1735 observations were made at St. Domingo, Panama, and Quito, using a plummet suspended by a thread of the aloe; about the same time the party sent to measure an arc of the earth within the polar circle swung a pendulum within twenty-four degrees of the pole. Lacaille carried a pendulum to the Cape of Good Hope and the Isle of France, Legentil took one on his voyage to the Indian Ocean, Phipps on his voyage toward the north pole, and Malaspina while visiting the Spanish possession in the Western hemisphere-Biot, Arago, and Borda were perfecting the pendulum and measuring gravity at different places in France; the labors of Ross, Kater, Foster, and Sabine were giving to England the supremacy in matters pertaining to gravity determinations; while Bessel, in Germany, was busy investigating corrections for the weight of air by swinging a pendulum in a vacuum, then in gases of known elasticity.
The French, not willing to follow in the lead of others, sent out expeditions under Freycinet and Duperrey, who brought back pendulum data that still find their places in the discussion of the earth's figure. These were followed up by Sawitsch in Russia, Plantamour in Switzerland, Basevi in India, and Peirce in the United States.
During all this time attention was given chiefly toward perfecting the mechanism of the pendulum without changing materially its form. It became heavier rather than lighter; the supports were correspondingly more cumbersome; the knife-edges subjected, because of increased weight, to greater danger of dulling, while theory was continually devising corrections because of atmospheric pressure and viscosity. The defects in structure took on an exaggerated magnitude, and the chance to discover absolute corrections appeared hopeless when the rapid advance in physical science set a limit of error to direct observation, and it looked almost as if the pendulum would be a doomed instrument of investigation.
Just in this emergency Superintendent Mendenhall, of the United States Coast and Geodetic Survey, called to his aid his experienced assistants to so modify the form of the pendulum as to bring it into its proper sphere of usefulness. Skilled as a physicist, it was not possible for him to waste time stumbling through the mistakes detected by the experience of others. He started anew where they had stopped.
The first point reached was the important one. By an application of the principle of coincidences first employed by Foucault in 1850 in determining the velocity of light, it became possible to as-
