C. GENERAL PHYSICS. 183 



of the Irene pondering it, I became conscious of the exceed- 

 ing power of the sun beating against my back and heating 

 the objects near me. Beams of equal power were foiling on 

 the sea, and must have produced copious evaporation. That 

 the vapor generated should so rise and mingle with the air 

 as to form an absolutely homogeneous medium I considered 

 in the highest degree improbable. It would be sure, I 

 thought^ to rise in streams, breaking through the superin- 

 cumbent air now at one point, now at another, thus rendering 

 the air flocculent with wreaths and strire, charged in different 

 degrees with the buoyant vapor. At the limiting surfaces 

 of these spaces, though invisible, we should have the con- 

 ditions necessary to the production of partial echoes and the 

 consequent waste of sound." This hypothesis Tyndall sub- 

 sequently tested. He found that the shielding influence of 

 a cloud, which cast a shadow from the fog-horn to the vessel, 

 caused an increase in the intensity of the sound. He also 

 observed that falling rain and fog, by rendering the air of 

 homogeneous density, produced the same effect. He then 

 proceeds to say: "But both the argument and the phenomena 

 have a complementary side, which we have now to consider. 

 A stratum of air less than three miles thick on a calm day 

 lias been proved competent to stifle both the cannonade and 

 the horn-sounds employed at the South Foreland; while, 

 according to the foregoing explanation, this result was due 

 to the irregular admixture of air and aqueous vapor, which 

 filled the atmosphere with an impervious acoustic cloud on 

 a day of perfect optical transparency. But, granting this, it 

 is incredible that so great a body of sound could utterly dis- 

 appear in so short a distance without rendering any account 

 of itself. Supposing, then, instead of placing ourselves be- 

 hind the acoustic cloud, we were to place ourselves in front 

 of it, might we not, in accordance with the law of conserva- 

 tion, expect to receive by reflection the sound which had 

 failed to reach us by transmission? The case would then 

 be strictly analogous to the reflection of light from an or- 

 dinary cloud to an observer placed between it and the sun. 



"My first care in the early part of the day in question was 

 to assure myself that our inability to hear the sound did not 

 arise from any derangement of the instruments on shore. 

 Accompanied by Mr. Edwards, who was good enough on this 



