154 ANNUAL OF SCIENTIFIC DISCOVERY. 



quantity be divided into the space through which the wave of sound is 

 known to travel in a second, we shall have the limit of perceptibility 

 in time. The foregoing plan is the most simple, but not the most 

 accurate, method of arriving at the quantity sought. The better plan 

 is to employ another person to produce the sound while the observer is 

 stationary at the distance of at least a hundred and fifty feet from the 

 wall. The person who produces the sound being placed between the 

 observer and the wall, at such a distance from the latter as to give a 

 distinct echo ; he is then directed gradually to approach the wall until 

 the echo and the direct sound become one. The distance measured 

 as before mentioned, will give the limit required. From a series of 

 experiments on this plan, Professor II. found the limit ot perceptibility 

 to vary from about GO to 80 feet ; or, in other words, the distance from 

 the wall at which the echo ceased was from 30 to 40 feet. This will 

 give from the one twentieth to the one fifteenth part of a second in 

 time for the ear to distinguish the difference of two sounds which fol- 

 low each other at an interval of one fifteenth of a second. 



The experiments, when made under the same circumstances, gave 

 the same result, almost within a single foot ; but when a different 

 source of sound was employed, and different observers, there was 

 observed the difference of results, giving the limits between one twen- 

 tieth and one fifteenth of a second. The limit was less with a sound 

 produced by an instrument which gave a sudden crack without perceiv- 

 able prolongation, such as is produced by an ordinary watchman's 

 rattle when made to emit a single crack. This difference may be 

 explained by taking into consideration the actual length of the sonorous 

 wave. If a sound occupies one quarter of a second, which is about the 

 time required for the utterance of a short single syllable, the length 

 of the sonorous wave will be about 300 feet, and hence, when the dis- 

 tance travelled by the two sounds is not more than 80 feet to and from 

 the wall, the two waves must overlap through a considerable portion 

 of their whole length, and will be only separated at the two extremi- 

 ties. The portion of overlapping may therefore determine the limit 

 of perceptibility, and this again is combined with the fact of the con- 



,. 1 L i* ii c j.i n f 



tinuance of a sonorous impression on the nerve ot the ear. rrojessor 

 Henry, American Association, Cincinnati. 



ON AIR-BUBBLES FORMED IN WATER. 



DR. TYXDALL, at the British Association, showed, by a few simple 

 experiments, that water falling in a continuous column, which it 

 always does for a certain distance, into another vessel of water, pro- 

 duces neither air-bubbles nor sound ; but that, as soon as the dis- 

 tance is so increased as that the end of the column becomes broken 

 into drops, both air-bubbles and sounds, varying from the hum of 

 the cascade and of the ripple to the roar of the cataract and of the 

 breaker, were produced. That the end of the column of issuing 

 water, although it only seems to waver, in consequence of a delusion 

 arising from the effect of the rapid succession on the retina, was really 

 composed of separate drops, the author said was proved by a very pretty 





