^So Sauffure's Dlaphanomeler. 



and again dire&ed your eves to the pafteboard, the circle will 

 be again viable, and you muft continue to recede till it dif- 

 appear once more. You muft then let your eyes reft afecond 

 time in order to look at the circle again, and continue in 

 this manner till the circle becomes actually invifible. 



If vou vvifti to find an accurate exprefficn for the want of 

 tnmfparencv, vou muft employ a number of circles, the dia- 

 meters of which increafe according to a certain progreflion, 

 and a comparifon of the diflances at which they difappear 

 will give the law according to which the tranfyareney of the 

 atmoiphere decreafes at different diftances. If you wi(h to 

 compare the tranfparency of the atmofphere on two days, or 

 ;n two different places, two circles will be fufficieut for the 

 experiment. 



According to thefe principles, M. de Sauffure caufed to be 

 prepared a piece of white linen cloth eight feet fquare. In 

 the middle of this fquare he fewed a perfect circle, two feet 

 in diameter, of beautiful black wool ; around this circle he 

 left a white ring two feet in breadth, and the reft of the 

 fquftre was covered with pule g) n. In the like manner, 

 and of the fame material-, he prepared another fquare; 

 which was, however, equal to only ,\ of the fize of the 

 former, fo that each fide of it was 8 inches ; the black circle 

 jn the middle was two inches in diameter, and the white 

 fpace around the circle was z inches alfo. 



If two fquares of this kind be fuipended vertically and 

 parallel to each other, fo that they may be both illuminated 

 in an equal degree by the fun ; and if the atmofphere, at the 

 moment when the experiment is made, be perfectly tranfpa- 

 rent, the circle of the large fquare, which is twelve times the 

 fize of the other, muft be fcen at twelve times the diftance. 

 In M. de Sauffure's experiments the fmall circle difappeared 

 at the difianec of 314 feet, and the large one at the diftance 

 of 3588 feet, whereas it mould have difappeared at the dif- 

 tance of 3768. The atmofphere, therefore, was not perfectly 



tranfpurent. 



