254 Dr. Wollaston on double linages, &c. 



where the refraction occasioned by heat must always be coun- 

 teracted by evaporation, never to exceed a few minutes; and 

 that any one in a situation commanding a view of the sea, by 

 attention to the various degrees of the dip of the horizon under 

 different circumstances, might soon form some estimate of the 

 proper allowance to be made, for brightness of the sun at the 

 time of an astronomical observation, or for difference of tem- 

 perature between the sea and air. 



Having now examined the several peculiarities of refraction 

 which I proposed for consideration, I shall, in few words, reca- 

 pitulate the purport of the foregoing pages. 



According to the theory here given, there appear to be two 

 opposite states of the atmosphere, either of which may occasion 

 objects to be seen doubled or tripled, since both increase and 

 decrease of its density, when partial, produce similar effects. 



It has been explained, 



ist. Why air heated by the moderate warmth of the sun's 

 rays, occasions objects to appear doubled and inverted. 



sdly. Why rarefaction, by a higher degree of heat, gives an 

 additional image, which is not inverted. 



3dly. In what state of evaporation the increase of the air's 

 density brings distant objects into view by unusual elevation. 



4thly. Under what circumstances evaporation may also pro- 

 duce an inverted image less elevated. 



And it is probable, that the same reasoning will afford a ready 

 explanation to other varieties of terrestrial refraction, that may 

 have been, or may hereafter be observed. 



