178 OF VERTIGO. SECT. XX. 5. 



larity : but there are fome peculiar circumftances attending this 

 effecl: of moving objects, 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 circum- 

 jacent objeds become quite indiftm6t, 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 himfeif by vifion, and begins to dagger, and attempts to 

 recover himfeif by his mufcular feelings This daggering adds 

 to the inilability of the vifible objedls by giving a vibratory mo- 

 tion befides their rotatory one The child then drops upon the 

 ground, and the neighbouring objefts feem to continue for forne 

 feconds of time to circulate around him, and the earth under 

 him appears to librate like a balance. In fome feconds of time 

 thefe fenfarions of a continuation of the motion of objects van- 

 i(h ; 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 pafling from Leith to Kinghorn in Scotland, 

 a fudden change of the wind (hook the undiftenJed fail, and 

 ftopt our boat ; from this unufual movement the pafiengers all 

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

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

 to be (ick , and 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 ; 

 and I am allured by a gentleman of obfervation and veracity, 

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

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

 ftantaneoully ceafed, and recurred again, when the danger was 

 over. 



Thofe, who have been upon the water in a boat or (hip fo 

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

 on that unftable element, at their return on land frequently 

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

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

 librate like the motion of the veflel. This 1 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 itage coach, 



