[ 4 ] 



them, and the elegance of his dedudion, ftill the truly phyfical 

 inquirer muft lament the circumftances of the obfervations them- 

 felves, which, it is not eafy to fuppofe, could afford him indifput- 

 able elements, when we confider that they were made in a con- 

 fined room, in which the temperature was in general widely 

 different from that of the external air, and by the help of inftru- 

 ments clofely attached to a mafs of ftone of nine or ten feet 

 fuperficial fquare by three or four feet in thickncfs. We have 

 good rcafon to fuppofe that fuch a bulk of cold ftone decom- 

 pofes the furrounding air to fome diftance. In fome particular 

 circumftances of the atmofphere, the moiftnre refting on the 

 furface, and in others, the tremulous motion of the adjacent air, 

 feem to indicate either a decompofition or furcharge in that por- 

 tion of the air, by means of which the theory of refradions has 

 hitherto been experimentally determined ; and therefore leave the ■ 

 fubjed ftill liable to objedions apparently well founded. 



The prefent Aftronomer Royal, Dodor Nevil Mafkelyne, 

 whofe fagacity lefs important matters could not efcape, aware 

 of this defed, has opened his Obfervatory more to the air, and, 

 as far as the conftrudion of the building would admit, has 

 removed part of the evil ; the quadrants, however, ftill remain 

 attached to the great mafs of ftone. 



Ever fince that important period, at which aftronomy appears 

 to have affumed a new face, by the introdudion of metallic 

 inftruments of more accurate frame and divifion, and the adoption 

 of telefcopic fights, the groffer difficulties of the fcience have been 

 gradually removed : fome, which before that time were confidered cf 



little 



