[ 5 ] 



little impDrtanpe, and others, 'till then totally unfufpeded, are 

 now beconie of ferious confetjuence, and require the mofl: 

 accurate theoretical invefligation, and the utmoft refinement 

 of pradice. Amongft thefe may be ranked the minute varia- 

 tions of refradion ; which may, perhaps, juftly be confidcred as 

 the greateft bar at prefent to the perfedion of Aftronomy. 



I MUCH fufped that the true conftitution of our atmofphere is, 

 even now, but little known ; but I am not without hopes of inte- 

 refting difcoveries in this important branch of fcience, from the novel 

 means of exploring its qualities in circumftances very different 

 from thofe of all former experiments. We are, however, certain 

 that it is fubjedt to decompofition and change ; and obfervation 

 and theory prove to us that refradions are thereby affeded. 



If we propofe by obfervation to procure elements for a law 

 of refradion, whether we adopt the conftitution of the atmo- 

 fphere fuppofed by our predecefTors or not, it ismanifeft that 

 obfervations. jnade. in the open air promife the greateft degree 

 of confiftency with each other, and the beft elements for a 

 theory and law of refradions ; but as this method, particularly 

 in our northern climates, cannot be purfued with fafety either 

 to the Aftronomer or his apparatus, we can only endeavour to 

 approximate to this perfedion, by making our buildings as open 

 to the air as may be confiftent with their particular ftrudure, 

 and the health and convenience of the obferver. This fubjed 

 had for a long time engaged my attention, and I had digefted 

 what occurred to me upon it into fome form, when, by a happy 



concurrence 



