276 OF VERTIGO. SECT. XX. 7. 



tually with irritative ideas of their apparent moti- 

 ons. The ideas of apparent motions are always 

 irritative ideas, becaufe we never attend to them, 

 whether we attend to the objects themfelves, or to 

 their real motions, or to neither. Hence the ideas 

 of the apparent motions of objefts are a complete 

 circle of irritative ideas, which continue throughout 

 the day. 



Alfo during all our waking hours, there is a 

 perpetual confufed found of various bodies, as of 

 the wind in our rooms, the fire, diftant converfa- 

 tions, mechanic bufmefs ; this continued buzz, as 

 we are feldotn quite motionlels, changes its loud- 

 nefs perpetually, like the found of a bell ; which 

 rifes and falls as long as it continues, and feems to 

 pulfate on the ear. Thus any one may experience 

 by turning himfelf round near a waterfall ; or by 

 finking a glafs bell, and then moving the direction 

 of its mouth towards the ears, or from them, as 

 long as its vibrations continue. Hence this undu- 

 lation of indiftincl: found makes another concomi- 

 tant circle of irritative ideas, which continues 

 throughout the day. 



We hear this undulating found, when we are 

 perfectly at reft ourfelves, from other fonorous 

 bodies befides bells ; as from two organ-pipes, 

 which are nearly but not quite in unifon, when 

 they are founded together. When a bell is ftruck, 

 the circular form is changed into an elliptic one ; 

 the longed axis of which, as the vibrations conti- 

 nue, moves round the periphery of the bell ; and 

 \vheneither axis of this eliipfe is pointed towards 

 our ears, the found is louder ; and lefs when the 

 intermediate parts of the ellipfe are oppofite to us. 

 The vibrations of the two organ -pipes may be 

 compared to Nonius's rule ; the found is louder, 

 when they coincide, and lefs at the intermediate 

 times. But, as the found of bells is the mod 



familiar 



