Introduction 



ally clear, not the least sign of a cloud in sight and the sun 

 was shining down with merciless intensity, making everything 

 hot that it struck. There was very little wind. Under 

 these conditions all the instruments were inaudible at a 

 distance of two miles. 



It then occurred to the Professor that there was an 

 acoustic cloud between the instruments and the ship, and he 

 reasoned it out in this way : when the sun shines on an 

 ordinary cloud, the cloud refuses passage to the light ; the 

 light is broken up and numerous reflections take place ; the 

 light is not all lost some of it is reflected. The side of a 

 cloud that the sun shines on is very bright ; why, then, 

 should there not be a similar result from an invisible acoustic 

 cloud ? The immense energy of the sound sent out must 

 go somewhere ; if the cloud refuses its passage, would it not 

 be sent off in the other direction ? Would it not be reflected 

 back as is the case with light on a common cloud ? As he 

 could get no response from the sound sent out, he thought 

 he would see what he could obtain from the other side of 

 the cloud. I quote the following: 



" It is incredible that so great a body of sound could utterly 

 disappear in so short a distance without rendering some account 

 of itself. Supposing, then, instead of placing ourselves behind the 

 acoustic cloud we were to place ourselves in front of it, might 

 we not, in accordance with the law of conservation, expect to re- 

 ceive by reflection the sound which had failed to reach us by 

 transmission? The case then would be strictly analogous to the 

 reflection of light from an ordinary cloud to an observer 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. Price Edwards at I p.m., I was rowed to the shore, and 



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