534 THE POPULAR SCIENCE MONTHLY. 



which observations could be made, in comparison with direct 

 measures of the earth, has caused it to be regarded as a most im- 

 portant geodetic instrument. 



As early as 1735 observations were made at St. Domingo, Pana- 

 ma, 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 posses- 

 sion 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 investigat- 

 ing 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 Rus- 

 sia, Plantamour in Switzerland, Basevi in India, and Peirce in 

 the United States. 



During all this time attention was given chiefly toward per- 

 fecting the mechanism of the pendulum without changing mate- 

 rially its form. It became heavier rather than lighter ; the sup- 

 ports were correspondingly more cumbersome; the knife-edges 

 subjected, because of increased weight, to greater danger of dull- 

 ing, 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 in- 

 vestigation. 



Just in this emergency Superintendent Mendenhall, of the 

 United States Coast and Geodetic Survey, called to his aid his ex- 

 perienced 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 applica- 

 tion of the principle of coincidences first employed by Foucault in 

 1850 in determining the velocity of light, it became possible to as- 



