146 TWINKLING OF 



formation of which a large quantity of caloric is 

 evolved, cannot be questioned ; and it is a fact, 

 familiar to almost every one, that the rarefied air 

 which, in serene, sunny weather in summer^ 

 ascends from the heated surface of the ground, 

 and mingles with higher strata of inferior warmth 

 and tenuity, gives to objects viewed through it, 

 a tremulous appearance, somewhat analogous to 

 that of twinkling ; indeed, the planets, whatever 

 their altitude may be, when seen through the hot 

 air which issues from the chimney of a furnace, 

 do positively twinkle very visibly : we may, 

 therefore, reasonably infer, as like results neces- 

 sarily ensue from the operation of the same 

 causes, that the twinkling of the stars, agreeably 

 to the laws of dioptrics, is occasioned by the 

 variable refraction produced by the blending to- 

 gether of volumes of air of diflferent temperatures 

 and densities. 



Regarded, a;lmost universally, as utterly inap- 

 plicable to any purpose of practical utility, the 

 twinkling of the stars attracts little attention, 

 except as a matter of mere curiosity ; yet, to the 

 intelligent meteorologist, it sometimes affords the 

 earliest indication of changes, which are con- 

 stantly occurring in regions of the atmosphere to 

 which he has no immediate access ; and it ena- 

 bles the attentive astronomer to ascertain, with 



