[ 14 ] 



■ wefterly winds prevail, I believe, moderately fpeaking, two-thirds 

 of the year ; and as this circumftance fecures us in that proportion 

 from any eft'edl of the fmokc of the city, fo this difpofition of the 

 meridian room frees us in the fame proportion from fmoke or other 

 vapours from the houfe or building. 



As this is the moft effential part of the work, it will require a 

 more particular detail. 



The meridian room is thirty-feven feet two inches long, and 

 twenty-three feet broad in the infide clear, and twenty-one feet 

 high. It is defigned for the ufual obfervations of the paffages of 

 the heavenly bodies over the meridian, and of their meridian 

 altitudes ; thefe effential objeds require the moft minute attention 

 in every particular. 



But as I do not mean in this paper to enter into a detail of 

 the particular inftruments, I fhall confine myfelf to an account 

 of the methods adopted to procure convenience of obfervation, 

 ftability and temperature. 



The broad crofs in figure 2d, plate II. reprefents a piece of 

 the moft folid mafonry, rifing from the rock to within a few 

 inches of the joifts of the floor, and totally unconneded with 

 the walls. At X,X is laid down a folid block of Portland ftone 

 of nine feet two inches in length, by three feet in breadth, and 

 one foot four inches thick. This block fupports the pillars of 

 the tranfit inftrument, whofc bafes are marked by X,X ; thefe 

 pillars are feven feet fix inches high, their bafes three feet 

 from North to South, and two feet fix inches from Eaft to 



Weft. 



