*9* O P V E R T I G O. SECT. XX. f, 



caufe we have been accuftomed to them both ; that 

 is, to undergo the one, and to fee the other at the 

 fame time. But in riding over a broad and fluc- 

 tuating ftream, 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 firii: we go on (hip-board, where the move- 

 ments of ourfelves, and the movements of the large 

 waves are both new to us, the vertigo is almoft un- 

 avoidable with the terrible ficknefs, which attends it. 

 And this I have been affured has happened to ft- 

 veral from being removed from a large fliip into q. 

 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 

 Jofe' our perpendicularity : but there are fome pecu- 

 liar circumstances attending this effeft of moving 

 qbje&s, which we come now to mention, and (hall 

 hope from the recital of them to gain fome infight 

 into the manner of their production. 



When a child moves round quick upon one foot, 

 the circumjacent objects become quite indiftincl:, as 

 their diftance increafes their apparent motions ; and 

 this great velocity confounds both their forms, and 

 their colours, as is feen in whirling round a many 

 coloured wheel ; he then lofes his ufual method of 

 balancing himfelf by vifion, and begins to ftagger, 

 and attempts to recover himfelf by his mufcuhr 

 feelings. This daggering adds to the inftability of 

 the vifible obje&s by giving a vibratory motion 

 befides their rotatory one. This child then drops 

 upon the ground, and the neighbouring objects 

 ieem to continue for fome feconds of time tq circu- 

 late around him, and the earth under him appears 

 to librate like a balance. In fome feconds of time 

 thefe fenfations of a continuation of the motion of 

 pbjefts yanifh 5 but if he continues turning round 



fome what 



