58 Ol'fervatlons upon an umifual 



vertical to a, an ereft image C, both of them very perfefit 

 and well defined, and an image v iv of the fea between them, 

 the water appearing very didinftly. As the (liip was coming 

 up towards the horizon, I continued to obferve it, in order to 

 dii'cover the variations which migbt follow, and fovmd, that 

 as the fliip approached the horizon, the image C gradually 

 difappcared, and at laft it vaniflied ; after that, the image 

 V ■Tu of the fea difappeared ; and during this time the image E 

 defcended ; but the fliip did not rife lb near to the horizon 

 as to bring the main-malls together. Had I direfted my 

 lelefcope to the fame point of the horizon a little fooner, I 

 fliould have feen the two images before the fliip itfelf was 

 vifible. In faft, the images were vifible when the whole 

 fliip was aAually below the horizon ; for, from the very 

 fmall part of the maft which was at firft vifible, that part 

 mufl; then have been below the horizon, and appeared above 

 it by the ufual lefraftion; the altitude of a, above the ho- 

 rizon, having then been much lefs than the increafe of alti- 

 tude which arifes from the common horizontal refra6lion. 

 The difcovery of fliips in this manner might, in fome cafes, 

 be of great importance; and, on fuch occafions, it might be 

 worth while to appoint proper perfons to make obfervations 

 for that purpofc. 



The clifl's at Calais being very vifible, T directed oiy tele- 

 fcope towards them, in order to examine whether there was 

 anything unufual in their appearance ; when I obferved an 

 image of the cliffs above the cliffs thcmfelves, together with 

 an image of the fea feparating them, as is reprefented in 

 Fig. 5.; in which, xy reprefcnts the horizon of the fea, AB 

 the cliffs, a b their image, and viu the imae;e of the fea be- 

 tween them : the depth of a b was much lefs than that of 

 AB. It is probable, however, that v iv might not be. the 

 image of the fea immediately adjoining to the cliffs, but a 

 partial elevation of tlic fea at fome dillance from them ; and 

 that the image viv might intercept fome part of the image 

 a b, which would othervvife have been vifible : we muft not> 

 therefore, conclude, that the image a b, fo far as it appeared, 

 was lefs than the correfponding part of the objeft. Froni 

 the memorandums which I made at the time of obfervation, 

 5 Ida 



