C. GENERAL PHYSICS. 163 



mission took place in January, 1871. Experiments were also 

 continued subsequently to the conclusion of the armistice, 

 and the details so perfected that at night-time the signals, 

 as they were then made, were read with the naked eye at a 

 distance of twenty-three miles, and the practical utility of 

 the method thus completely demonstrated. 6 B^ 1873, 36. 



HAEMONIC ECHOES. ' 



Lord Rayleigh communicates the explanation of certain 

 interesting echoes, which he calls harmonic echoes, a name 

 given to it by Dr. Brewer. On the river Nahe, not- far from 

 Coblentz, is an echo which makes thirteen repetitions at cer- 

 tain intervals. Sometimes the echo seems to approach the 

 listener, sometimes to be retreating from him ; sometimes dis- 

 tinct, sometimes feeble ; at one time it is heard to the right, 

 at another to the left. The special peculiarity of it consists 

 in that the sound heard is at times in unison with the direct 

 sound, and at others it is a third, fifth, or tenth of the funda- 

 mental sound. Similar echoes, but perhaps more musical, 

 are found at Paisley, Scotland, and at the Lake of Killarney, 

 in Ireland. The latter is celebrated in that it returns in re- 

 sponse to any simple air played on a bugle a very excellent 

 repetition of it, having a pitch corresponding to the second 

 harmonic. An echo near Glasgow returns any note that 

 may be played a third lower, and after a few minutes' pause 

 a second repetition is heard still lower than .the former, and 

 after a similar pause the same notes are repeated a third 

 time in a still lower and feebler tone. Rayleigh, while hesi- 

 tating to believe that these descriptions are not somewhat 

 exaggerated, thinks that they have a basis of truth, and has 

 himself observed the sound of a woman's voice echoed from 

 a plantation of fir-trees, but with the pitch raised an octave. 

 The explanation which he gives of these interesting phenom- 

 ena presumes that the echo is returned to the hearer, not 

 from a plain surface, but from a broken surface, or a group 

 of small obstacles, from the surface of each of which a small 

 wave of sound is reflected back to the observer. This dif- 

 fused sound he considers would necessarily contain the higher 

 elements, that is, the notes of higher pitch, in excessive pro- 

 portion, and, consequently, the direct wave, being shorn of 

 these higher elements, will appear duller than the original 



