W. B. Taylor— Recent Researches in Sound. 41 



That this very interesting subject presents features requir- 

 ing still further and more refined investigation is sufficiently 

 obvious from the single consideration that aerial opacity and 

 echo have not been shown to bear that direct relation which 



that this was our day of longest echoes, and it ^ 

 day of greatest acoustic transparency, the association suggest- 

 ing that the duration of the echo is a measure of the atmo- 

 spheric depths from which it comes. On no day, it is to be 

 remembered, was the atmosphere free from invisible acoustic 

 clouds ; and on this day when their presence did not prevent 

 the direct sound from reaching to a distance of 15 or 16 

 nautical miles, they were able to send us echoes of 15 sec- 

 onds duration." {Trans., p. 202.) If these echoes were not 

 "folded," this would represent an extreme limit of about a 

 mile and a half. Our most powerful sounds cannot afford to 

 waste much of their energ)- on echoes, if under the inexorable 

 law of increasing attenuation as the square of the distance 

 they are to be audible through a range of 16 miles : less than 

 the 400th of the intensity at one nautical mile, that is heard 

 at the distance of 100 yards from the source ; and one 25Hth 

 of this at the distance of 16 nautical miles, or less than the 

 hundred thousandth of the intensity at 100 yards. And the 

 inference is strong that in such a case accompanying echoes 

 must be derived from sound beams in a somewhat different 



Further observations are needed also to ascertain whether 

 these aerial screens of unequal density and acoustic opacity 

 are capable of returning echoes on opposite sides, as is to be 

 expected if we may accept the analogy of catoptrics : and 

 whether the echoes are as frequently heard from steamers in 

 mid-ocean, or whether they mainly attach themselves to coast 

 lines. As Professor Henry has well stated: "Much farther 

 investigation is required to enable us to fully understand the 

 effects of winds on the obstruction of sound, and to determine 

 the measure of the effect of variations of density in the air due 

 to inequality of heat and moisture." {L. H. Rep., p. 117.) 



As the last of the series here selected, Tyndalfs observation 

 [8] agrees well with the observation of Duane [1]. 

 [To be concluded.] 



