SECT. XX. 3. OF VERTIGO, t ?7 



ftantly lofes his perpendicularity, and tumbles on the ground. 



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

 our own real movements from the apparent motions of objects. 

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

 ftructs us with accuracy to difcern thefe modes of motion, and 

 to afcribe the apparent motions of the ambient objects to our- 

 felves ; but thofe, which we 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 carelefsJy attend to diftant hills or woods 

 through a thin hedge, which is near us, we obferve the jumping 

 and progreffive motions of them 5 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 ;ot 

 accuftomed to, the objects feem to undulate, a$ he pafles, 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 us, 

 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 other at the fame time. 

 But in riding over a broad and fluctuating ftream, though we; 

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

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

 And when firft we go on (hip-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 hap- 

 pened to feveral from being removed from a large ihip into a 

 imall one ; and again from a fro -ill one inro a man of war. 



5. From the foregoing example:* it is e^ :-1eir, that, when we 

 are furrounde,d with unufual motions, we lofe our perpendicu- 



VOL. I. hrity 



