Royal Astronomical Society. 241 



22 h 8'" 35 s . Mr. Lassell's estimation of distance corresponds to a 

 greatest elongation of about 17'; which, as well as the period, agrees 

 so nearly with the result of the Poulkova observations, as to render 

 it highly probable that the satellite observed by Mr. Lassell on 

 Sept. 27, 1845, is the same as that observed by M. O. Struve. 



Extract of Letter from M. Octo von Struve to the Astronomer 

 Royal. 



" You will see from the Astronomische Nachrichten, &c, that we 

 have not been idle at Poulkova since your visit. In addition to the 

 published accounts I have little to say, except that my father's cal- 

 culations of the great Russian meridian arc give a considerably larger 

 value to the difference between the two axes of the earth than has 

 been hitherto found. I cannot tell you the exact quantity, as the 

 calculations are not completed. 



"I have finished my observations of the satellites of Uranus for 

 this season. As soon as I have a little leisure, I shall deduce their 

 motions from the observations. 1 have seen the third satellite, di- 

 stinctly, only once since my communication to Sir John Herschel, 

 viz. at 6 p.m. on January 25th. Its position was 202°, its distance 

 about 18'. I am now inclined to think that the differences of the 

 light of this satellite in different parts of its orbit are so great, that 

 it cannot be seen by our refractor when it is in the opposite direction 

 to that in which I have hitherto observed it. The atmosphere was 

 very favourable on two occasions, when the satellite was supposed 

 to be to the north of the planet, but I could not see the least trace 

 of it. The period of its revolution is, however, somewhat uncertain, 

 for these negative observations are far from conclusive. 



" M. Dollen has finished his calculations on Bessel's fundamental 

 catalogue for 1820. The result, as respects Procyon, is that the 

 irregularity supposed by Bessel in its proper motion, vanishes 

 altogether." 



May 12, 1848. — On an easy Method of approximating to the di- 

 stance of a Planet from the Sun by means of two observations only, 

 made near the Planet's opposition, By Professor Chevallier. 



If v be the linear velocity of the earth, and r the distance of the 

 planet from the sun, the earth's distance being 1, the linear velocity 



of the planet = —7-. 

 Vr 



Also the angular retrograde velocity of the planet at or near op- 

 position 



_ 1 / _ v \ 

 r— 1\ Vr) 



Vr(Vr+l)' 



Again, let L, L' be the heliocentric longitudes of the earth at the 

 times of observation, and /, /' the geocentric longitudes of the planet, 

 and let 



_ L'— L _ heliocentric velocity of the earth 

 I— I' geocentric velocity of the planet' 



