of Binaural Audition. 269 



by experiments which may be very readily made with respect 

 to this point. 



The cause of this, however, lies in the particular circum- 

 stances which unconsciously influence the estimate of the di- 

 rection. The most important of these circumstances are : — 



(1) Trains of thought suggesting a certain direction as 

 being that in which a source of sound is constantly situated. 



To give one single example : — Suppose while walking along 

 a road a person hears himself called by name, he will most 

 naturally conjecture that the person calling is behind him; and 

 conjectures thus, not only for the reason that he does not see 

 the speaker in front of him, but also specially because the very 

 purpose of so calling is usually to bring to a standstill the 

 person called, so that he may be more easily overtaken. 



(2) A certain indistinctness which generally characterizes 

 the indirect perceptions of sound. This arises from the cir- 

 cumstance that the indirect- sensations of sound are nothing 

 less than a whole series of echoes following one another, and 

 more or less running into one another, producing a lengthen- 

 ing-out of every single element of sound, so that the .last 

 echoes of one element of a sound may coincide with the first 

 echoes of the element immediately following it. 



Thus, for example, in hearing sounds which come from 

 behind upon an open plain, as in figure 10, the sound of the 

 voice of the speaker 8 (the arrows indicate the line of sight) 

 can only reach the hearer H by reflexion at the surface of the 

 earth, which is never perfectly level. Hence a sound of in- 

 finitely short duration proceeding from S will be drawn out 

 to a considerable length, since the rays of sound reflected from 

 the more distant points of the earth's surface do not arrive 

 simultaneously but reach the hearer in rapid succession. 



Now it may happen that either, as is the case when the 

 earth has an equably rough surface, the later echoes are weaker, 

 or, as when there are reflecting surfaces, such as hills or houses 

 present, the later echoes are stronger than those which precede 

 them. 



The total impression of an indirectly perceived momentary 

 sound, such as a crack, will in the former case be a dying out 

 or ceasing to sound, in the latter case an irregular decreasing 

 and swelling again with a rolling effect. 



This last phenomenon is perceived when thunder is heard, in 

 which all the sudden claps occurring during the progress of a 

 prolonged peal imply each a reflecting surface such as a moun- 

 tain, causing great volumes of sound simultaneously reflected 

 to arrive at certain irregular intervals. Similar phenomena 

 occur also after the firing of a cannon in a mountainous neigh- 

 bourhood. 



