292 0?i the rciodty of UgJit, [July 



seconds after the actual time of its occurrence. But to 

 proceed — The fact of the progressive motion of light was 

 first brought to our knowledge by Roemer who found that 

 the eclipses of the satellites of Jupiter happened sooner 

 or later in the exact proportion of the distance of the sa- 

 tellite from the earth ; and at a later date the observa- 

 tions of Bradley upon the fixed stars (which l)y reason 

 of the earth's motion in her orbit causes their places to 

 appear advanced in the direction in which the earth 

 travels) has confirmed not only the fact, but the numeri- 

 cal accuracy of the determination ; the exact determina- 

 tion of the velocity of light it must be recollected is not 

 our present enquiry, since it v.'ili lead to formula and fi- 

 gures which are foreign to our present purpose ; to sup- 

 ply however in an imperfect way the want of these ; we 

 will now proceed to enuuierate some of the most impor- 

 tant disturbing causes vvhich affect our enquiry. As far 

 back as 1600 Kepler had discovered that the motion of 

 the earth and other planets were not performed in cir- 

 cular orbits, but ellipses ; and that the area passed over 

 by a line joining the sun and planet was always propor- 

 tional to the time, or in other words that the motion was 

 more rapid in the orbit the nearer the planet was situat- 

 ed to the sun — what observation had pointed out to 

 Kepler as the phenomena of nature, the theory of New- 

 ton and subsequent Geometers has proved to be the con- 

 sequences of the laws of universal gravitation — It must 

 be noticed however, that this unequal m.otion of the pla- 

 nets in their orbits undergoes its changes in the time of 

 one complete revolution of the planet ; or to be more 

 particular, if the planet Jupiter (which now more imm^e- 

 diately concerns us) in his circuit round the sun per- 

 forms a certain space in any one day, on the 10th 

 June, 1820 for ei' ample it will perform very nearly an 

 equal space on the IGth April, 1832 when it has complet- 

 ed one revoliiiion or come io the same point in its orbit. 



