rOL. XII.] PHILOSOPHICAL TRANSACTIONS. SQ/ 



bored through with a taper-bore, and a basket may be used at the bottom of 

 the pump instead of the box colander. 



On the Motion of Light. By M. Romer* N° 136, p. 893. 



Philosophers have been endeavouring for many years to decide, by some 

 experiment, whether the action of light be conveyed in an instant to distant 

 places, or whether it requires time. M. Romer of the Royal Academy of 

 Sciences has devised a way for this purpose, taken from the observations of the 

 first satellite of Jupiter, by which he demonstrates, that for the distance of 

 about 3000 leagues, which is nearly equal to the diameter of the earth, light 

 needs not one second of time. 



Let A (fig. 13, pi. 11) be the sun, B Jupiter, C the first satellite of Jupiter, 

 which enters into the shadow of Jupiter, to come out of it at D; and let 

 EFGHLK be the earth placed at divers distances from Jupiter. — Now suppose 

 the earth being at L, towards the second quadrature of Jupiter, has seen the 

 first satellite at the time of its emersion or issuing out of the shadow at D ; and 

 that about 424^ hours after, viz. after one revolution of this satellite, the earth 

 being in K, see it returned to D ; it is manifest, that if the light require time 

 to traverse the interval LK, the satellite will be seen returned later to D, than 

 it would have been if the earth had remained at L; so that the revolution of 

 this satellite being thus observed by the emersions, will be retarded by so much 

 time, as the light shall have taken in passing from L to K; and that on the 

 contrary, in the other quadrature FG, where the earth by approaching goes to 



* Olaus Romer, or Roemer, a noted Danish astronomer and mathematician, was born at Arhusen 

 in Jutland, l644. Having acqviired great skill in those sciences, when M. Picard was sent by 

 l/ouis XIV to make observations in the north, in l67\, he was so much pleased with this young 

 man, that he engaged him to return with him into France, where the king settled a pension on him, 

 and honoured him with the appointment of mathematical preceptor to the Dauphin : he was also 

 joined with Picard and Cassini, in making astronomical observations) and in l672 he was admitted a 

 Member of the Academy of Sciences. During the ten years he resided at Paris, he acquired great 

 reputation by his discoveries, and among them that above noticed, concerning the progressive motion 

 of light. Yet it is said he complained afterwards, that his coadjutors ran away with the honour of 

 many things that belonged to him. Probably this induced him to return into his own country, which 

 he did in 168I, where he was appointed to the office of mathematician to the King, and astronomi- 

 cal professor, with a large salary. He afterwards was honoured with several other offices of dignity, 

 particularly that of counsellor of state, and burgomaster of Copenhagen. Roemer was preparing to 

 publish the result of his observations when he died, in 1710, at 66 years of age. This loss however 

 was supplied by his pupil, Peter Horrebow, professor of astronomy at Copenhagen, who published 

 them, with his method of observing, in 1735, under the title of Basis Astronomicae. Several of his 

 pieces were also printed in the Memoirs of the Academy of Sciences at Paris, particularly in vol. i 

 and X of the collection of \666. 



