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XVIII. On the Lunar Atmosphere. By Thomas Firminger, 

 LL,D, 



At is said in the Number of the Philosophical Magazine for June, 

 (vol. 53. p. 465) that Mr. J. B. Enimett observed an occulta- 

 tion of a small star by the moon on the fifth of December 1818 ; 

 and that he saw the star when *' really behind the moon's disc." 

 It would, 1 think, afford some satisfaction to many of your astro- 

 nomical readers to know how this gentleman ascertained that the 

 small star spoken of, was actually behind the moon's disc at the 

 time he saw it; he could not 1 think be assured of it by observa- 

 tion only, and as to calculation, the place of the moon is not so 

 accurately computed as to assure him that the star was actually 

 behind the moon when he saw it. The slow motion of the sun, 

 in rising or setting, and the great refractive power of our atmo- 

 sphere, enable us to know, from calculation, that he is below the 

 horizon, when visible above it; but these circumstances do not 

 take place at the moon. I have observed in occultations of stars 

 at the moon's bright limb, that their light diminishes as they ap- 

 proach towards the moon, and in a few seconds before the oc- 

 cultation they appear very small, and seem to vanisii gradually. 

 A very remarkable instance of this circumstance I observed 

 whilst I was assistant to Dr. Maskelyne, and which is recorded 

 in the account of the Greenwich Observations : but 1 always con- 

 sidered this appearance to have arisen from the superior bright- 

 ness of the moon to that of the star, when very near its enlight- 

 ened limb ; the apparent magnitude of the star being rendered 

 almost a point at the instant of its disappearance ; but when the 

 star emerges at the moon's dark limb, it emerges with almost its 

 full splendour. The appearance is also the same when the star 

 immerges at the dark limb behind the moon. Whether the star 

 has immerged or emerged at the moon's dark limb, the appear- 

 ance has been always instantaneous in all the occultations that 

 I have seen. In all the eclipses of the sun yet recorded, the cir- 

 cular section of the sun, formed by the moon's limb, is always re- 

 gular and well defined; which I think would not be the case, had 

 the moon an atmosphere sufliciently dense to occasion a refrac- 

 tion so great as Mr. Emmett observed. Whether the occulta- 

 tion he has recorded took place at the moon's enlightened or 

 dark limb does not appear in the journal ; but I think, from the 

 collective testimony of my own observations, that Mr. Emmett 

 was led to conclude, that the star was behind the moon's limb, 

 from some deceptive appearance. In the instance I have alluded 

 to, the star did not appear to vanish instantaneously, but gra- 

 dually to disappear behind the moon's enlightened liml). I how- 

 ever never suj)poscd this appearance to have been caused l)y re- 



G 3 fraction. 



