40S On a new and certain MeLkod of ascertnin'mg the Figure 



and for many seconds to appear not only adhering to the cir- 

 cuniference, but oftentimes projected on the disc, of the moon. 

 Whether this optical deception is caused by an atmosphere sur- 

 rounding the mooii, or by the instrument used for the observa- 

 tion*, or by what other means, I shall not stop to discuss : but, 

 as it is probable that its effects may Ije more discernible in those 

 peculiar occultations which liave l)een the subject of the preced- 

 ing Memoir, it is desirable that the particular circumstances of 

 the case in each occultation should be noted down as they occur ; 

 together with a description of the telescope used for the observa- 

 tion. Astronomers indeed now adopt the method of noting down 

 the moment of apparent contact, as well as the moment of the 

 disappearance of the star : and when it emerges again from be- 

 hind the moon, they likewise note down not only the moment of 

 its reappearance, but also the moment of its separatio7i from the 

 moon's disc. Indeed every remarkable phenomenon which may 

 occur during cmy observation ought to he carefully registered: as 

 it is only.bv a comparison of such cases that we can ultimately 

 expect to discover the causes which influence or produce them. 



M. Cagnoli, in his Memoir, seems to have addressed his re- 

 marks chiefly to the principal observatories of Europe. But, 

 however desirable it may be to have the co-operation of those 

 scientific bodies, it is evident that many private individuals may 

 do much towards accomplishing the object and design of the au- 

 thor. In many cases, they may perhaps be able to do more than 

 can be effected in any fixed observatory: since they may be more 

 favourably situated, on the earth's surface, for the observation, or 

 may enjoy the advantage of a more clear and favourable atmo- 

 sphere. Indeed, the greater the number of oi)servers attached 

 to this duty, the greater will be the chance of an accurate result: 

 for, independent of taking a mean of the observations made near 

 the same spot, we might be enabled to detect the errors of the 

 lunar tables bv means of simultaneous oI)servations made in a fa- 

 vourable situation in a distant part of the world ; agreeably to 



come ivhite : and that for 10" before its actual immersion it appeared ad- 

 hering to the border of the moon. Connaissance des Terns, ^liim'e 1810, 

 page i)3G. In a subsequent occultation of the same star, on July 1 1, 1757» 

 he states that, just before its immersion, also on the illuminated side of the 

 moon's disc, it appeared adhering to the border of the moon for 7"> and af- 

 terwards remained 2" on the moon's disc, previous to its total disappearance : 

 and that the star was easily discernible on the face of the moon by a reddinh 

 colour which it had ac(iuired on approaching thereto. Ibid, page 339. 



* In the occultation of Aldcharan on July 5, 1755, Messier, with a 4j 

 feet Newtonian reflector magnifying 140 times, sav/ the star for near 2" on 

 the moon's disc : La Gcntil, with a refracting telescope of () or 7 feet, saw 

 it adharinu^ to the border of the moon, for 4" or 5" : whilst Cassini, with his 

 IS feet telescope, saw it detached from the border, to the very moment of 

 immersion. Ibid, page 335. 



what 



