'262 On pccuiiar Noises occasionally heard 



are divided into two waves, where the medium suddenly changes, 

 and a sort of acoustic mirage is produced, arising from the 

 want of homogeneity of the air, in the same manner as a lumi- 

 nous mirage takes place from an analogous cause *. But, ad- 

 mitting the ingenious explanation of this scientific traveller, 

 other causes possibly, however, and probably connected with 

 the presence or absence, excess or diminution, of solar heat, may 

 be operative in both the increase, and protracted continuity of 

 sound. Thus, Captain Sir Edward Parry, during the intense cold 

 experienced in Winter Harbour, was surprized at the great dis- 

 tance at which the human voice could be heard : " I have," he 

 says, " often heard people distinctly conversing in a common 

 tone of voice, at the distance of a mile, and to-day I heard a 

 man singing to himself as he walked along the beach at even 

 a greater distance than this." The strong tendency of sound 

 to ascend, again, has great effect. Humboldt remarks the bark- 

 ing of a dog has been heard when the listener was at an eleva- 

 tion of about three miles in an aerostatic balloon. And it has 

 been remarked, that, from the edge of the Table Mountain, 

 which is 3600 feet high, and the upper part of which rises per- 

 pendicularly at the distance of about a mile from Cape Town, 

 every noise made below, even to the word of command on the 

 parade, may be distinctly heard. 



The conducting power of water is well known, but to what 

 extent would scarcely be credited, had we not the most un- 

 doubted evidence at hand, that of the much to be lamented Dr 

 Clarke, whose words we shall give. " A remarkable circum- 

 stance occurred, which may convey notions of the propagation 

 of sound over water, greater than will perhaps be credited ; but 

 we can appeal to the testimony of those who were witnesses 

 of the fact, for the truth of that which we now relate. By our 

 observation of latitude, we were 100 miles from the Egyp- 

 tian coast ; the sea was perfectly calm, ■with little or no swell, 

 and scarcely a breath of air stirring, when the Captain 

 called our attention to the sound, as of distant artillery, vibrating 

 in a low gentle murmur, upon the water, and distinctly heard 

 at intervals during the whole day. He said it was caused by 

 " Ann. de Chim. vol. xii. p. 162 ; also this Journal, old series, vol. iii. p. 194. 



