NOTES. XXXI 



satellites (Newton died about half a year before Bradley's discovery of aberra- 

 tion), 7' 30" from the Sun to the Earth, and assuming a distance of 70 

 millions of English (statute) miles, light traverses, in every second of time, 

 155555^ English miles, the reduction of which to geographical miles 

 would vary according to the assumption of the figure of the Earth. Accord- 

 ing to Encke's exact assumptions in the Jahrbuch for 1852 (taking, with 

 Dove, 1 English mile = 5280 English feet=4954'206 Paris feet), there are 

 691637 English statute miles to an equatorial degree. Newton's result would 

 thus be 33736 German geographical miles 15 to a degree, (or 134944 English 

 geographical miles 60 to a degree, which are generally used throughout 

 this translation under the name of " geographical miles"). But Newton took 

 the Sun's parallax at 12" : if it is 8"'57116, as given by Encke's calculation 

 of the transit of Venus, the distance is greater, and we should have for the 

 velocity of light (taking 7i' from the Sun) 47232 German, or 188928 

 English geographical miles for a second of time, too much, therefore, instead 

 of, as before, too little. It is certainly very remarkable, though it was not 

 noticed by Delambre (Hist, de 1' Astronomic moderne, T. ii. p. 653), that 

 whereas, from Homer's discovery in 1675 to the beginning of the 18th 

 century, the times assigned for the passage of light over half the major axis 

 of the Earth's orbit fluctuated between 11' and 14' 10", being always much 

 too high, Newton, supported perhaps by more recent English observations of 

 the first satellite, came within about 47" of the truth (or, at least, of the now 

 accepted result of Struve). The oldest memoir in which Romer, who was 

 Picard's pupil, presented his discovery to the Academy, bears date Nov. 22, 

 1675. He found, by forty emersions aud immersions of Jupiter's satellites, 

 " un retardement de lumiere de 22 minutes par 1'intervalle qui est le double 

 de celui qu'il y a d'ici au Soleil" (Memoires de 1'Acad. de 16661699, T. x. 

 1730, p. 400). Cassini did not contest the fact of the retardation, but he 

 contested the assigned amount of time, because (he very erroneously supposed) 

 different satellites gave different results. Du Ham el, the Secretary of the 

 Paris Academy (Regise Scieutiarum Academies Historia, 1698, p. 145), 

 seventeen years after Romer had left Paris, but still referring to him, gives 

 from 10' to 11'; but we know, by Peter Horrebow (Basis Astronomies, sive 

 Triduum Roemerianum, 1735, p. 122 129), that in 1704 six years, 

 therefore, before his death when about to publish his own work on the 

 velocity of light, Romer kept steadily to the result of 11' : so, also, did 

 Huygens (Tract, de Lumine, cap. 1, p. 7). Cassini proceeded quite diffe- 

 rently : he found for the first satellite 7' 5", for the second 14' 12"; and he 



