[ i8 ] 



For this purpofe the meridian apertures for the tranfit inftrument 

 and circle are fix feet wide ; which is a breadth confider- 

 ably greater than I have yet heard of in any Obfervatory. 

 Thefe, or a part of them, fhould be left open until the tem- 

 perature within and that abroad are found to agree entirely, 

 or as nearly as can be efFeded : through thefe and the win- 

 dows there is a free admiffion of air ; but to break the force 

 of the wind, which might agitate the plumb-lines, and at times 

 difplace the inftrument, there are fkreens of the thinneft canvas 

 pervious to the air, which are contrived, occafionally to cover the 

 aperture, except a fpace of two feet in the middle ; but this only 

 ufed in windy weather. And that the temperature within may 

 at all times be more nearly equal to that of the external air, 

 Plate II. there are femicircular air-holes in the walls grated and covered 

 ^'S- 3- ^j^}^ ^}^g fame kind of wide canvas, which are defigned 

 to be left always open to the air, except in wet or damp 

 weather, at which times they may be clofcd with fhutters within. 

 The fame provifion is made here as in the equatorial dome to pre- 

 vent the tranfmiflion of heat. 



The South and North wings now only remain to be explained. 



The South wing is defigned for occafional obfervations, fuch 

 as eclipfes, occultations, &c. which being confined to the planets, 

 require only the range of that part of the hemifphere in which 

 thefe can at any time be vifible. On this account the centre 

 of the fouthern dome is fo for removed towards the South, that 

 a line drawn from thence to the extremity of the meridian room 



fliall 



