The Evolution of the Sciences 



storm and invades the plain with a speed of 

 nearly one kilometre per second. 



We have said that for a total eclipse it is 

 necessary that the length of the cone formed by 

 the moon's shadow should exceed the distance 

 separating us from the moon; if, as frequently 

 happens, this condition is not fulfilled the 

 eclipse will be only annular in the most favoured 

 regions ; in such cases the solar disc will, even at 

 the most favourable moment, project all round 

 the moon, and darkness will never be complete. 



All these phenomena can be predicted without 

 difficulty by astronomers, because the move- 

 ments of the heavenly bodies are known with 

 the greatest precision. At present the pre- 

 dictions of the ephemeridae are given to the 

 tenth of a second, but this degree of approxi- 

 mation has only been obtained gradually and 

 by an increasingly accurate knowledge of the 

 constants of the solar system. At the commence- 

 ment of the nineteenth century eclipses were 

 foretold with an approximation of only several 

 seconds; a hundred years earlier La Hire's 

 tables contained errors of 4 to 5 minutes, and 

 before the appearance of these tables the pre- 

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