144 Vncommon EffeSis of Ref ration upon 



importance ; and on fuch occafions, it might be worth while to appoint proper perfons to make 

 obfervations for that purpofe. 



The clifFs of Calais being very vifible, I diredled my telefcope towards them, in order to ex- 

 amine whether there were any thing unufual in their appearance; when I obferved an image of 

 the cliffs above the clifFs themfelves, together with an image of the fea feparating them, as is 

 reprefented in fig. 5, in which x y reprefents the horizon of the fea, A B the clifFs, a h their 

 image, and v lu the image of the fea between them ; the depth of a ^ was much lefs than that 

 of A B. It is probable, however, that v w might not be ihe Image of the fea immediately ad- 

 joining to the clifFs, but a partial elevation of the fea at fome diflance from them ; and that 

 the image v w might intercept fome part of the image a b, which would otherwife have been 

 vifible ; we muft not, therefore, conclude that the image a b, fo far as it appeared, wasiefs than 

 the correfponding part of the objeft. From the memorandums which I made at the time of 

 obfervation, I do not find that I e>:amined the appearance of the clifF, A B, and its images 

 a b, which had there, at that time, been any flriking marks in them, would have determined 

 whether the objeft and its image were of the fame magnitude. The image a b was, however^ 

 (Breft ; the boundaries on the top of A B and a b agreeing together. Having examined this 

 <br fome time, and taken a drawing of the appearance, during which I could difcover no 

 variation, I diredled my telefcope to other objects ; and upon turning it again to the fame 

 clifFs, after the fpace of about fix or feven minutes, the images a b and v w were vanifhed: 

 but examining them again foon after, the images were again vifible, and in every refpe(5t the 

 fame as they appeared before. A fliort time after they difappeared, and did not appear any 

 more. 



Soon after the above appearances, I obferved a fliip, C, with the hulk below the horizon, x y, 

 paffing by the fame clifFs, A B ; an inverted image, D, of which appeared againfF the clifFs, 

 as reprefented in fig. 6. The fhip was in motion, and remained at the fame diflance on the 

 other fide of the horizon. I continued my obfervations upon it, till it had pafFed the clifFs for 

 a confiderable diflance, but there was no change of appearance. The clifFs were illuminated 

 by the fun, and appeared very diflindly; but there was no image above, as in the lafl cafe. 



Continuing to obferve the fame clifFs, A B, fig. 7, I foon after difcovered two partial ele- 

 vations, ot «, of the fea, by the unufual refraction ; they changed their figures a little, and 

 difappeared in the place where they firfF appeared, and were equally diflinft in every part. 



About this time, I difcovered a very thick fog coming upon the horizon from the other fide, 

 rolling upon it with a prodigious velocity, curling as it went along, like volumes of fmoke 

 Ibmetimes out of a chimney. This appeared feveral times : I conclude, therefore, that there 

 was a confiderable fog on the other fide of the horizon. 



Thelaft phenomenon which I obferved, was that which is reprefented in fig. 8, where x y 

 reprefents the horizon, a b two partial elevations of the fea meeting at c, and continued to d; 

 ty another partial elevation of the fea, of which kind I obferved feveral, fome of which moved 

 parallel to the horizon with a very great velocity. I conjecture, therefore, that thefe ap- 

 pearances were, in part at leaft, caufed by the fog on the other fide the horizon ; for though I 



ilid 



