XI 



and motion of the node of Venns. The difficulties raised in the theory of 

 Mercury, although not removed, were slightly diminished by the same 

 increase of the earth's mass. 



" In his solar tables, M. Le Verrier has adopted the value 8*95" for the 

 mean equatorial horizontal solar parallax, this value was obtained by deter- 

 mining from observation the coefficient of the lunar equation, and assuming 

 the mean lunar parallax and data furnished by the theories of precession and 

 nutation. 



" The way in which M. Le Verrier has thus evolved from the theories of 

 Venus, the Earth, and Mars, the necessity of the value of the mean solar 

 parallax much greater than the usually received value 8' 57", and not differing 

 greatly from 8*95", must render it extremely probable that the true value of 

 the sun's mean parallax does not differ greatly from that quantity." 



Stone then proceeds to discuss the Greenwich and Willi amstown 

 observations, and derives a value of 8'932" for the solar parallax. 



In the 'Monthly Notices' for May, 1863, Stone remarks that in 

 the liiuar theories of Plana, Pontecoulant, and Lubbock the co- 

 efficient of the parallactic inequality deduced with the usually 

 received values of the involved constants amounts to 122* 1", which, 

 if we increase the value of the mean solar parallax by a thirtieth 

 part, becomes 126*2", coinciding closely with the observed values as 

 derived by Airy, viz.: — 124*7" from meridian and 125*5" from altazi- 

 muth observations. He suggests that it would be a point of interest 

 to determine whether Hansen's lunar theory would bear any con- 

 siderable increase in the mean solar parallax. 



Stone had apparently overlooked a letter of Hansen's,* in which 

 the writer says : — 



" The coefficient of the parallactic equation I found to be 



125-705", 



" an amount exceeding any which has hitherto been assigned, and which 

 indicates a greater value of the sun's parallax than has been deduced from 

 the observations of the transit of Venus. The Greenwich observations, 

 exclusive of any others, assign the foregoing value of the parallactic in- 

 equality, and the Dorpat observations nearly the same value. I cannot, 

 therefore, alter it." 



In the following number of the ' Monthly Notices, 'f Hansen, in 

 reply to Stone, refers to the above quoted paragraph, and in the 

 same periodical for November, 1863, gives with more detail, as the 

 result of his researches, the value 8*9159" for the mean solar 

 parallax. 



Till the note of alarm was thus sounded by Hansen in 1854, and 

 echoed by Le Verrier in 1861, astronomers had almost universally 



* ' Monthly Notices R.A.S.,' May, 1854. 

 t June, 1863. 



