OBSERVATIONS ON VENTRILOQUISM. 123 



that found is molt effective, when its courfe is molt directl y 

 oppofed towards the organ ; and (hews by experience, that the 

 head itfelf is a fenfltive folic!, capable of perceiving founds by 

 their action upon its furface. Hence when the differences of 

 effect of the fame found, from its difference of diftance and 

 direction with regard to the two ears, and the two fides or 

 portions of the head are confidered, there will be a fufficiently 

 great variety in the fenfations to mark the circumftance of 

 direction, and produce an habitual power of difcrimination in 

 this refpect as in all others, which continually affect our daily 

 operations. 



The annexed diagram, Fig. 2, Plate VII. is given to ex- Diagram to (hew 

 plain the phenomena which arife from the pulfes of found * he eftefts ° ( 



r . t r *T; found on the 



being obftructed by the hearer's head, as they move in the head, 

 horizontal plane pafiing through his ears, which cafe ought to Direft hearing. 

 be treated feparately from the more complex one that com- 

 prizes the angle of elevation, along with the horizontal dif- 

 tance from the axis of hearing. When the fonorous object 

 fiands directly in front of the hearer, the femicircle A C B may 

 be fuppofed to reprefent the horizontal fection of his head, 

 pafling through the places of the ears, E and F, and the axis 

 of hearing E F ; alfo let G be the place of the founding body, 

 which, according to the conditions of the cafe, lies in the 

 plane A C B produced, and likewife in the right line G S, 

 which bifects E F at right angles ; feeing then E F is bifected 

 by the perpendicular S G, the arch ECF is alfo bifected by 

 the fame in the point C. Draw L G, G K, to touch the cir- 

 cle in T and P, then will the arcs TEC, and C F P be equal. 

 Now all the pulfes which do not move in right lines, con- 

 tained in the angles T G S and SG P fly off without touching 

 the circle ; confequently they add nothing to the found im- 

 preffed on the ears by the body placed at G, whether the 

 places E and F be fuppofed to lie in the arcs T C and C P, 

 or without them. But the fame number of pulfes equal in 

 force will fall in a given time, and in fimilar directions, on the 

 arcs TEC and C F P as well as on the ears fituated at E and 

 F ; and it is equally manifeft, that the fame reafoning will 

 apply to two equal and fimilar folids, conftructed upon the 

 equal and fimilar planes, ECS and F C S. Now founds 

 though it be formed in the ears, is very much increafed by the 

 vibrations excited in the contiguous parts of the head by the 



pulfes 



