1 7 8 Of VERTIGO. Sect. XX. 5. 



larity : but there are fome peculiar circumftances attending this 

 effect of moving objects, which we come now to mention, and 

 fhall 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 circum- 

 jacent objects become quite indiftinct, 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 bal- 

 ancing himfelf by vtfion, and begins to dagger, and attempts to 

 recover himfelf by his mufcular feelings. This daggering adds 

 to the inftability of the vifible objects by giving a vibratory mo- 

 tion befides their rotatory one. The child then drops upon the 

 ground, and the neighbouring objects feern to continue for fome 

 teconds of time to circulate 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 objects van- 

 i(h j but if he continues turning round fomewhat longer, before 

 he falls, ficknefs and vomiting are very liable to fucceed. But 

 none of thefe circumftances affect thofe who have habituated 

 themfelves to this kind of motion, as the dervifes in Turkey, 

 amongft whom thefe fwift gyrations are a ceremony of religion. 

 In an open boat pafFmg from Leith to Kinghorn in Scotland, 

 a fudden change of the wind fhook the undiflended fail, and 

 ftopt our boat ; from this unufual movement the pafTengers all 

 vomited except myfelf. I obferved, that the undulation of the 

 fhip, and the inftability of all vifible objects, inclined me ftrongly 

 to be fick -, an4 this continued or increafed, when I clofed my 

 eyes, but as often as I bent my attention with energy on the 

 management and mechanifm of the ropes and fails, the ficknefs 

 ceafed *, and recurred again, as often as I relaxed this attention 5 

 and I am affured by a gentleman of cbfervation and veracity, 

 that he has more than once obferved, when the veflel has been in 

 immediate danger, that the fea-ficknefs of the pafTengers has in- 

 ftantaneoufly ceafed, and recurred again, when the danger was 

 over. 



Thofe, who have been upon the water in a boat or fhip {o 

 long, that they have acquired the neceflary habits of motion up- 

 on that unliable element, at their return on land frequently 

 think in their reveries, or between lleeping and waking, that 

 they obferve the room, they fit in, or fome of its furniture, to 

 librate like the motion of the veflel. This I have experienced, 

 myfelf, and have been told, that after long voyages, it is fome, 

 time before thefe ideas entirely vanifh. The fame is obfervable 

 in a lefs degree after having travelled fome days in a ftage coach, 



and 



