66 GREAT MEN OF SCIENCE 



of water on the earth takes place mainly on and over 

 the surface, via the whole atmosphere. Further, Mariotte 

 was the first to develop accurate ideas concerning the forma- 

 tion of raindrops in clouds, from the vapour rising up from 

 the solid earth and from the sea. 



OLAUS ROEMER 



1644-iyio 



His greatest achievement was the first proof of the finite 

 velocity of light, and an actual measurement of this velocity. 

 He was born at Aarhus, in Denmark, and there studied 

 science and mathematics. From his twenty-eighth to his 

 thirty-seventh year, he was employed at the observatory in 

 Paris, where he was particularly interested in the movements 

 of the moons of Jupiter discovered by Galileo. He noticed 

 in the case of one of the moons, the inmost, that its period of 

 revolution, measured sharply by its entry into the shadow of 

 Jupiter, appeared to be of variable magnitude, according to 

 the position of the earth in its orbit. When the earth was 

 more distant from Jupiter, the moon appeared to be late; in 

 the other half of the year, when the earth is again nearer to 

 Jupiter, the loss of time was made good. This connection 

 led Roemer, after concluding his observations over a period 

 of several years, to work out an idea already familiar to 

 Galileo, namely that light takes time for its propagation. 

 He therefore assumed that in his observations the late arrival 

 of the light coming from Jupiter was perceptible, since it 

 had to follow up the earth which was in rapid motion, and 

 he therefore calculated the velocity of light as the quotient 

 of the semi-diameter of the earth's orbit and the total re- 

 tardation in a half year. The result expressed in our present 

 day units and based upon more accurate observation, is 



