The Operation and Use of the Apparatus 



sun shining on the surface of the water is constantly rising and 

 its place being taken by the colder air above. It is like the 

 rush of the third-class passengers through the rushing first class 

 at a railway station. This of course produces a great many 

 conflicting currents of aii of different densities, certainly very 

 favourable for the production of acoustic clouds ; optically the 

 air is clear and transparent. At two o'clock we send out blasts 

 in various directions, and, although there is nothing in sight, 

 we receive responses of considerable power, showing unmistak- 

 ably the presence of invisible acoustic clouds. The readings 

 on the paper strip, however, are drawn out and much longer 

 than the primary blast. Acoustic clouds never appear in foggy 

 weather ; they are produced by many conflicting currents of air, 

 of different densities, the result of bright and unobstructed 

 sunshine. 



The last three days of our voyage are cloudy, rendering 

 observation impossible. It is a very dark and foggy night, 

 and we are approaching the rugged coast of Ireland. We work 

 our vibrator constantly, sending out strong blasts every few 

 seconds, but we receive no response until near morning. We 

 know that there is no danger, as our apparatus shows that the 

 coast is clear, so we keep on full speed until we receive some 

 response to our blasts. The first is very feeble and comes from 

 a distance ; twenty minutes later the record is strong and 

 distinct, and there is more than one record. Some objects are 

 only three miles distant and of no great size, whilst others are 

 indicated fully ten miles distant, and from the size and shape of 

 the records must be of great size. It is the uneven coast of 

 Ireland, with mountains in the background. The fog continues 

 dense ; still, with our apparatus we are able to skirt the coast, 



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