438 



HISTORY OF SCIENCE. 



mers, and the distance between Dover and Falmouth having been 

 obtained in the trigonometrical survey of England, the length of a 

 degree of the parallel in that longitude was found by the difference of 

 time between the two places, and this was determined by carrying 

 chronometers between them. Another and much longer arc of longi- 

 tude was measured by the 

 French and Italians on the 

 Continent. Captain Sabine 

 conducted a valuable series 

 of pendulum observations in 

 various latitudes, and these 

 permitted the relative in- 

 tensity of gravity at different 

 places to be accurately de- 

 termined. The results of all 

 these measures of arcs is to 

 give to the earth an ellip- 

 ticity of about -g-^-, some 

 persons fixing on a number 

 a little greater, others on one 

 a little less. The pendulum 

 experiments give, by a 

 theorem of Clairaut's, an 

 ellipticity somewhat greater 

 than this. 



In this place we may de- 

 scribe the pendulum experi- 

 ments devised by Foucault 

 in 1 850, by which the earth's 

 rotation on its axis was in a 

 manner made visible to the 

 eye. The experiment de- 

 pends on the fact that a 

 freely suspended body when 

 made to oscillate will, in the 

 absence of any disturbing 

 force," continue to perform 

 its oscillations in one plane. 

 The application of this 

 principle to the exhibition 

 of the earth's rotation will be readily seen from Fig. 195, showing a 

 model in which a body is suspended at a point a in the line of the 

 axis of a globe representing the earth. The pendulum will continue 

 to make its vibrations in one plane when the globe is turned on its 

 axis as shown in the figures. If such a pendulum could actually be 

 erected at the earth's pole, and set to oscillate in a certain plane, that 



