[ I^ ] 



pillar itfelf, or the piers which are to arife from it, for the 

 fupport of the axis of the inftrument ; and for this purpofe 

 the floor is framed fo as to let the piers pafs through un- 

 touched ; whence no motion of the floor or furrounding wall 

 can be communicated to the inftrument, and the temperature 

 of the pillar is in fome meafure preferved by the furrounding 

 wall. The turning dome is framed of wood, each rib confifts 

 of three equal and parallel pieces, the grain of the wood in 

 thefe being fo difpofed as to counterad each other's inclination 

 to caft or warp. The frame is covered with canvas foaked 

 with drying oil, tar, and white paint, and coated afterwards 

 with white paint, wrought to fuch confiftence as to be laid on 

 with a trowel. The infide is to be covered with another thinner 

 fheeting, and between the two. wood-mofs, if neceflTary, will be 

 infroduced, to prevent the tranfmifllon of heat. The aperture 

 for obfervation is two feet fix inches wide, and opens to fix 

 inches beyond the zenith. As the dome is an hemifphere, the 

 Aide which fliuts this aperture is made to move vertically through 

 the zenith, with a movement fimilar to that of fome modern 

 writing-tables: The Aide pafling through the zenith defcends 

 through the oppofite quadrant of the hemifphere within the 

 dome ; but as this Aide exceeds ninety degrees of the hemi- 

 fphere by fix inches, it would be impoflible in opening the aper- 

 ture to make the Aide defcend through its whole length on the 

 oppofite fide, as' the wall-plate muft flop it before its afcending 

 extremity could reach the zenith : Therefore, to remedy this, a 

 tranfverfe hinge is crontrived in the Aide about eight inches 

 from that extremity, which thus defcends, or that end which is 

 next the zenith when the aperture is fliut ; this hinge lets thefe 

 eight hinges hang perpendicular to the horizon, when the Aide 



has 



