. XX. 3. 4. OF VERTIGO. 27* 



3. A fecond difficulty we have to encounter is to 

 diftinguifh our own real movements from the appa- 

 rent motions of objects. Our daily praclice of walk- 

 ing and riding on horfeback foon inftructs us with 

 accuracy to difcern thefe modes of motion, and to 

 afcribe the apparent motions of the ambient objects to 

 ourfelves ; 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 oc- 

 cur to us, appear at reft ; we can meafure their dif- 

 tances with our eye, and regulate our attitude by 

 them : yet if we carelefsly attend to diftant hills or 

 woods through a thin hedge, which is near us, we 

 obferve the jumping and progreffive motions of them ; 

 as this is increafed by the parallax of thefe objects ; 

 which we have not habituated ourfelves to attend to. 

 When firft an European mounts an elephant fixteen 

 feet high, and whole mode of motion he is not ac- 

 cuftomed to, the objects feem to undulate, as he 

 pafles, and he frequently becomes fiek and vertigN 

 nous, as I am well informed. Any other unufuai 

 movement of our bodies has the fame eftect 5 as riding 

 backwards in a coach, fwinging on a rope, turning 

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

 thoufand others. So after a patient has been lopg 

 confined to his bed, when he firft attempts to walk, 

 he finds himfelf vertiginous, and is obliged by prac- 

 tice to learn again the particular modes of the appa- 

 rent motions of objects, as he walks by them. 



4. A third difficulty, which occurs to us in learn- 

 ing to balance ourfelves by the eye, is, when both 

 ourfelves and the circumjacent objects are in real 

 motion. Here it is neceffary that we mould be ha- 

 bituated 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, without confounding his 

 jumping and progreflive motion with our own, be- 



caufe 



