PHYSICAL AKfD JLITERARY, 25 



I thought ifmight be worth while to fay 

 fomething in explication of it. The eafieft 

 way to be fatisfied of the matter experimen- 

 taUy> is, to hold a hair or down immediately 

 above the focus of a his or fpeculum^ or, to 

 blow a ftream t)f fmoke from a pipe hori- 

 zontally over it ; for, if the air in the focus 

 were hotter than the furrounding fluid, ife 

 would continually afcend upon account of 

 its rarefadion, and thereby fenlibly agitate 

 thefe flender .bodies. Or a his may be fo 

 placed as to form its focus within a body of 

 water or fome other tranfparent fubftance, 

 the heat of which can be examined from 

 time to time with a thermometer : but care 

 muft be taken in this experiment to hold the 

 lem as near as poflible to the tranfparent bo- 

 dy J left the rays, by falling clofTer than or- 

 dinary on its furface, fhould warm it more 

 than the common fun-beams. 



1 5. It is well known that the rays of light, 

 by pafling .obliquely thro' our atmofphere,- 



are 



366. of the aftronomical p<trt; arid No/et U^am He fhyjlqtie, 

 torn. 4. The filence of moft phyfical writers, con- 



cerning this paradoxical truth, makes it probable that tlic/ 

 were unacquainted with it. . 



