THE BLUE-RIDGE, WARM-SPRING, Sec. 223 



When k is confidered that in meafuring a height by- 

 taking the vertical angle, the refradtion of the rays of 

 light in the atmolphere mud i\ffctl the appearance of the 

 objedts, it will occur that this mode muft be fubjedl to 

 fame inaccuracy ; and as this rcfradlion generally tends 

 to increafe the apparent height, a reafonable allowance 

 on the experiments mentioned by Mr. Jefferfon would 

 probably bring them down to the barometrical meafure- 

 ment. This obfervation is beautifully illuftrated by Mr. 

 Jefferlbn's account of a phenomenon refembling, in fome 

 meal'ure, an appearance which feamen call looming ;* and 

 which (fo far as relates to apparent fize at leaft) may be 

 accounted for by refradlion. On the other hand baro- 

 metrical meafurement, is probably inaccurate in the 

 lower ftages of the atmofphere ; but this tends to leJJ'en 

 th e apparent height : The truth may therefore lie 

 betweeri. 



I am, as before, 



Philadelphia. 7 ^^^"^ ^''^ ^^"^^^^^^ y°^^'^' 



Aug. 18, 1796.5 JONATHAN WILLIAMS. 



To Mr. R. Pdtierfon. 



* Jederibn's Notes, ^ge 8f . 



F f 2 Mifcellanemis 



