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Ihall clear the greateft amplitude of any of the planets in this lati- 

 tude ; for as to comets, they are always moft conveniently ohferved 

 by the equatorial inftrument, and that has the entire horizon at 

 command in this Obfervatory. 



In this fouthem wing P, P reprefent the bafes of two pillars 

 which are to rife into the dome ; and, lefling on folid mafonry, 

 unconneded with the floor and furrounding walls, are to fupport 

 a polar axis, carrying an achromatic telefcope, to which it is pro- 

 pofed to apply an helioftatic movement, which will carry it round 

 with an equable motion in a fidereal day. This is not a matter 

 of fimple amufement, many advantages arife from fuch an appa- 

 ratus. Every pradifed obferver is aware of the errors that muft 

 neceffarily arife from the permanency of the efted of light upon 

 the organ of fight ; this makes it probable that we do not always 

 fee a ftar in its true place in the field of a fixed telefcope. In 

 the tranfit inftrument, for inftance, the image of the wire does 

 not change its place in the eye, but is permanent ; as that of a 

 ftar approaches this wire, the laft impreffion remains until the 

 ftar has pafled the wire, and before the new impreffion is fenfible. 

 As to the diftindnefs of vifion to be expeded from it, the follow- 

 ing experiment will be fufficient for any one who doubts. Let a 

 fine be drawn horizontally upon a wall, at fuch diftance from a 

 telefcope as that the obferver ftiall be able to read through it tole- 

 rably fmall print ; let the telefcope be direded to a certain pomt 

 in this line ; let a man move a printed paper, having both large 

 and fmall charaders along this line as equably as he can ; when 

 the paper comes oppofite the telefcope, the obferver will find that 

 the greater charaders alone are diftind, but let him follow the 

 paper with the telefcope and he will find the fmall charaders 

 ^ ^ C 2 diftind 



