ON ASTRONOMY. 



91 



to make it beat seconds, and on returning to Paris it was necessary 

 to restore the original length in order again to beat seconds. 



Picard had anticipated that a change of temperature would affect 

 the rate, but by no means to so great an extent as two minutes and 

 twenty-eight seconds per day. 



The cause of so remarkable a fact of course engaged the attention 

 of astronomers. 



Newton and Huygens both assigned the true cause, viz : the dimin- 

 ished force of gravity as a consequence of the centrifugal force and 

 the flattened figure of the earth. 

 The effect of the centrifugal 

 force may be illustrated by fig. 4. 

 Let E P Q S be a section of the 

 earth made by a meridian circle, 

 P S the axis, E Q the equator, 

 and P C Q an imaginary tube 

 extending from the pole to the 

 centre, and thence to the equa- 

 tor. Let this tube be filled with 

 water and the earth at rest. 

 The two branches would then be 

 of equal length. Now let the 

 earth begin to rotate on the line 

 P S. The water would imme- 

 diately settle down in P C and 

 rise in C Q. 



Newton found the ratio of the two axes to be as 229 : 230. There 

 could be no doubt that the figure w^s flattened pt the pole, the only 

 question was lioio much? 



6. To settle a question of so much interest and of so much importance, 

 the French Academy determined on extending the arc of Picard. 



In 1683, under the direction of the eminent astronomer royal, Do- 

 minic Cassini, the measurement was extended to some distance south 

 of Paris, when the death of the minister, Colbert, suddenly inter- 

 rupted their labors. They were not resumed till 1701, wlien the line 

 was extended to Perpignan, on the Mediterranean, G° south of Paris. 

 In 1718 James Cassini, son of the former, continued the measure 

 ment on the north of Paris to Dunkirk. Thus was this 

 completed after thirty years of labor. 



The unlooked for result was that a degree at the northern extremity 

 of the arc was lio fathoms shorter than at the southeo-n. 



The necessary consequence was that the earth was elotigated, not 

 fattened, at tlie poles, and that the conclusion of tlie matliematicians 

 was contradicted by the facts. 



This opinion, in opposition to those of Newton and Huygens, was 

 maintained by Cassini de Thury and his able coadjutors in the 

 Academy. To confirm so important a result, Cassini measured an 

 arc on a parallel of latitude extending from Strasburg to Brest, and 

 found it 300 toises shorter than it would have been if the earth liad 

 been a perfect sphere. In an elaborate work published by him in 

 1735 he says, as quoted by Delambre, (Hist. Ast. 18th Century, p. 



great work 



