Solar Eclipse of July 28, 1851. 7? 



form of a partial eclipse ; but, having himself witnessed a 

 total eclipse, he was able to assure them that no degree of 

 partial eclipse up to the last moment of the sun's appearance 

 gave the least idea of a total eclipse, as regarded either the 

 generally terrific appearances, or the singular nature of some 

 of the phenomena. Many years ago, in reading the admir- 

 able essay in the Philosophical Transaction by the late Mr 

 Baily on the eclipse (usually called that of Thales), the 

 occurrence of which suspended a battle between the Lydians 

 and the Medes, he had been struck by the cogency of Mr 

 Baily's arguments, which shewed that only a total eclipse 

 could be admitted as sufficient to produce the effect ascribed 

 to it ; and by the remark (cited by Mr Baily) of Maelaurin 

 and Lemonnier, that in an annular eclipse of the sun, even 

 educated astronomers when viewing the sun (nearly covered 

 by the moon) with the naked eye could not tell that it was 

 not full. The appearances, however, in a total eclipse, as 

 he should afterwards mention, were so striking, that there 

 could be no difficulty in believing the historian's account to 

 be literally correct. 



Proceeding first to explain the simple causes of 'a solar 

 eclipse, the Lecturer remarked that the moon's distance 

 from the earth is nearly one four-hundredth part of the sun's 

 distance, and that the moon's diameter is very nearly one 

 four-hundredth part of the sun's diameter, and that there- 

 fore, on the average, the sun's apparent diameter and the 

 moon's apparent diameter are very nearly equal. But in 

 consequence of the elliptic forms of their orbits, the sun's 

 distance is liable to small variation, and the moon's distance 

 to very considerable variations : when the moon is at the 

 most distant part of her orbit, her apparent diameter is 

 smaller than the sun's, and if she happens at that time to 

 be between a spectator and the sun, she will be seen as a 

 black disk covering the central part of the sun and leaving 

 a ring of light all round : when the moon is at the nearest 

 part of her orbit, her apparent diameter is larger than the 

 sun's, and she will, to a spectator in the proper locality, 

 completely cover the sun, and produce a Total Eclipse. But 

 neither of these things can happen unless the plane of the 



