()74 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



not depth enough of uniformly saturated atmosphere for the object itself to be seen 

 through it, but its true place, compared with that of the images, is represented 

 at a. 



Exper. 11. When I made use of rectified spirit in the same apparatus, I had 

 also sufficient proof that the laws of evaporation would admit of such appearances 

 being produced; for the same object now appeared curved downwards, as in fig. 12, 

 so that rays nearer to the bottom were manifestly less refracted than such as passed 

 at some distance above. A degree of convergency must therefore have been pro- 

 duced, though the distance at which the rays would meet, was beyond that of my 

 eye, and circumstances would not admit of my removing beyond 35 feet. 



The evaporation of water could not be expected to produce any sensible effect 

 of this kind, in so short a space ; but, in a view of some miles extent, there can 

 be no doubt, from the foregoing experiments, that evaporation from the surface 

 of the sea, in such a state of the atmosphere as would allow the lower strata to be 

 saturated, is capable of occasioning all the phenomena which have been described, 

 and probably was the cause of those which Mr. Vince observed. Since heat alone 

 tends to depress objects, and evaporation produces apparent elevation, it is proba- 

 ble, that in the instance of refraction related by Mr. Dalby, Phil. Trans, for 1795, 

 the heat of the sun was the principal agent, and that the moisture rather tended to 

 counteract than assist its action. Simple inversion may generally be seen, when 

 the sun shines on a dry even road of -f or ± mile extent ; but when the ground 

 has been wet, I have very rarely seen it, and have even failed of discerning it, 

 when the heat has been sufficient to raise a steam from the ground. The follow- 

 ing experiment shows that it is not to be expected but by very great extent of 

 surface. 



Exper. 12. I placed a dark-coloured board in the sunshine, and, having examined 

 the refraction along its surface, I made a wet lipe along it, with a sponge dipped 

 in boiling water. Notwithstanding this additional heat, the refraction, in the di- 

 rection of the wet line, was far less than over the rest of the board, though I took 

 care to observe the effect before the surface could be cooled again by evaporation. 

 I should therefore expect the depression of the horizon at sea, where the refrac- 

 tion occasioned by heat must always be counteracted 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 differ- 

 ence of temperature between the sea and air. 



Having now examined the several peculiarities of refraction which I proposed 

 for consideration, I shall, in few words, recapitulate the purport of the foregoing 

 pages. According to the theory here given, there appear to be 2 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 simi- 



