VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 673 



experiments, I tried whether confining the saturated atmosphere, by boards on 

 each side, would vary the effect, and found the refraction in all cases much 

 lessened; and, when water was used, it became imperceptible; but as soon as the 

 boards were removed, and a free current allowed to pass across, the full effect was 

 again produced. The reason of this difference appears to be, that the quicker 

 evaporation increases the degree of cold, and the current brings greater differences 

 of density contiguous. This state of rapid evaporation will fully account for the 

 phenomenon witnessed by Mr.Latham, who has described (in Phil. Trans, for 179^,) 

 an extraordinary elevation of the opposite coast of France, so as to be seen from 

 the beach at Hastings, and other parts of Sussex. There is a fact of the same 

 kind stated by De la Lande, (Astron. torn. 2,) who says that the mountains of 

 Corsica, though at the distance of more than 100 miles, are occasionally visible from 

 Genoa. It is probably owing to the same cause, that other objects have been 

 sometimes seen, at such distances that we should expect them to be intercepted by 

 the curvature of the earth ; for it is evident, that whenever the evaporation over 

 each mile of surface occasions a refraction of about 1 minute, the rays receive a 

 curvature equal to that of the ocean, so that its surface will appear flat, and the 

 spherical form of the earth will not obstruct horizontal vision of objects at any 

 distance. 



It still remained to explain the phenomena seen by Mr. Vince, as I had not 

 hitherto made an atmosphere capable of exhibiting images inverted, as well as ele- 

 vated, by increased density. For, in the refractions produced in the 7th, 8th, 

 and 9th experiments, by evaporation at an exposed surface, I observed the effect 

 was always greatest in contact with the evaporating surface ; any lower point a, 

 fig. 10, appeared brought nearer to a higher point c, by the pencils of rays from a 

 being more refracted at b, than the pencil from c was refracted at d. Therefore, 

 any rays passing from the eye at e, as a point, through b and d, would be made 

 to diverge to a and c; consequently visual rays could not, under these circumstances, 

 intersect each other, and no objects could appear inverted. But, whenever the 

 lowest strata of the air becomes saturated with moisture, the variations between the 

 saturated stratum and the incumbent atmosphere of the common density, will 

 follow a law similar to what is found at the confines of other fluids of unequal 

 density; hence, inversion will become visible, as there will be a point below which 

 the increment of density will decrease, and where the refractions will consequently 

 be less, though through a denser medium. 



Exper. 10. To produce these appearances, I procured a trough of thin deal, 

 5 feet long, 1 inch wide, with sides 24- inches high, and closed the extremities of 

 it with glass. A section of it is given in fig. 1 1 . When the bottom was wetted 

 with ether, the greatest refraction was, at intervals, more than %■ of an inch from 

 the bottom of the trough ; and beneath this height I saw a 2d image inverted, 

 when my eye was removed to 14 or 15 feet distance, and the object at about 70 

 feet. The focus seemed at the same time to be about 9 feet distant. There was 



VOL. XVIII. , 4 R 



