Sect, XX. 3. OF VERTIGO. 1 7 7 



ftantly lofes his perpendicularity, and tumbles on the ground. 

 3. A fecond difficulty we have to encounter is to diftinguifti 

 our own real movements from the apparent motions of objects. 

 Our daily practice of walking and riding on horfeback foon in- 

 ftru&s us with accuracy to difcern thefe modes of motion, and 

 to afcribe the apparent motions of the ambient object's to our- 

 felves ; but thofe, which we have have not acquired by repeated 

 habit, continue to confound us. So as we ride on horfeback the. 

 trees and cottages, which occur to us, appear at reft ; we can 

 meafure their diftances with our eye, and regulate our attitude 

 by them ; yet if we carelefsly attend to diitant hills or woods 

 through a thin hedge, which is near us, we obfervethe jumping 

 and progreffive motions of them j as this is increafed by the 

 parallax of thefe objects ; which we have not habituated our- 

 felves to attend to. When firft an European mounts an ele- 

 phant fixteen feet high, and whofe mode of motion he is not 

 accuftomed to, the objects feem to undulate, as he paries, and 

 he frequently becomes fick and vertiginous, as I am well inform- 

 ed. Any other unufual movement of our bodies has the fame 

 effect:, as riding backwards in a coach, fwinging on a rope, turn- 

 ing round fwiftly on one leg, fcating on the ice, and a thoufand 

 others. So after a patient has been long confined to his bed, 

 when he firft attempts to walk, he finds himfelf vertiginous, and" 

 is obliged by practice to learn again the particular modes of the 

 apparent motions of objects, as he walks by them. 



4. A third difficulty, which occurs to us in learning to balance 

 ourfelves by the eye, is, when both ourfelves and the circumja- 

 cent objects are in real motion. Here it is neceflary, that we 

 (hould be habituated to both thefe modes of motion in order to 

 preferve our perpendicularity. Thus on horfeback we accurately 

 obferve another perfon, whom we meet, trotting towards lis, 

 without confounding his jumping and progreffive motion with. 

 Our own, becaufe we have been accuftomed to them both j. that 

 is, to undergo the one, and to fee the oiher at the fame time. 

 But in riding over a broad and fluctuating dream, though, we 

 are well experienced in the motions of our horfe, we are liable 

 to become dizzy from our inexperience in that of the. water. 

 And when firft we go on fhip-board, where the movements of 

 ourfelves, and the movements of the large waves, are both new 

 to us, the vertigo is almoft unavoidable with the terrible fick- 

 nefs, which attends it. And this I have been aflured has has- 

 pened to fev'eral from being removed from a large fhip into a 

 fmall one ; and again from a fmall one into a man of war. 



5. From the foregoing examples it is evident, that, when we 

 are furrounded with unufual motions, we lofe our pcrpsndicu- 



VoL. I. ; Z iaritv- 



