I ca Elevation of Ohje&s above the Horizon. 



phoenomcnon : without the aftion of the fun's beam?, the 

 vapours -.vould not be denfe enough to conceal the objcfts : 

 it is by means of the folar rays that llicy are made lo dlfap- 

 pear from the eve. Thefe rays, however, can be tranfmittcd 

 to the eve onlv under a given angle, in tlie fame manner as 

 a certain angle is required in order lo produce the rainbow by 

 refraftion. The ftratum of air which is fufiiciently denfe to 

 effeft this reflcftion of the fun, rifts to a certain height, but 

 it can be perceptible to the eye only when the vapours ac- 

 tually throw back a great number of rays. . s foon as the 

 point of fight corrcfponding to this effeft is obtained, the eye 

 fees only the fliining vapour, which it cannot diftinguilh 

 from the heavens. This difappearance, however, takes place 

 in regard to diftant as well as to near nbjeols, and rather at 

 the lower than the upper part of the oljjeft, hccaufe, in the 

 .firft cafe, the direftion of the line of fight paifes through a 

 larger mafs of vapour, and in the fecond through a denfer 

 ftratum. The leis denfe part of the vapour, which did not 

 refleft rays of the fun, but which always grew denfer the 

 lower my ftation became, acting like a veil, which interpoied 

 itfelf between my eve and the trees which had not yet difap- 

 peared, made me obfcrve their diftancc, and cnnlributed to 

 caufe the elevated trees to appear asif floating behind the row. 

 In regard to the circumilancc that the tops of the diftant 

 trees dicl not raife themfelvcs above thofc of the foremoil:, 

 though their lower pans feemed to rife above the ground as I 

 flefccnded, and that thefe tops on the contrary fcemcd iomcr 

 what dcprefied ; it arofe from the decreafe in the elevation 

 of my point of ftation having a greater ratio to the lefs di- 

 ftant than to the moft diftant trees ; by which means, the 

 former were raifcd more in the fame proportion above the 

 horizon. 



When this phccnomcnnn is obfervcd on the banks ui 

 large pieces of water, and when the objecls are at difiereiit 

 diltances from the obferver, the truth of the principles here 

 advanced may be confirmed by remarking, that the phano- 

 menon is modified bv a change of pofitiou ; but if the ob- 

 ferver can move about in a horizontal direction, it will be of 

 great ufe to obferve the changes of the objcft in a vertical 

 direfition, fuppofing he has opportunity, on any gradual de- 

 clivity, of aiccndaig and delcending. ' If the vertical mea- 

 furement of the objects then changes, fo that their total 

 height decreaies the more the objects feem to rife from the 

 ground when the obferver defcends the declivity, this is a 

 fufficient proof that the whole image is not elevated, but that 



its 



