\ 



caused by atmospherical Refraction. 253 



tance 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 phaenomena 



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 probable, that in the instance 

 of refraction related by Mr. Dalby, (Phil. Trans, for 1795, 

 p. 587), 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 

 upon a dry even road of i or \ mile extent; but, when the ground 

 has beep 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 following experiment shews 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 line along it, with a sponge dipped in boiling water. Not- 

 withstanding this additional heat, the refraction, in the direction 

 of the wet line, was far less than over the rest of the board, al- 

 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, 



