Introduction 



landed at the base of the South Foreland cliff. The body of air 

 which had already shown such extraordinary power to intercept 

 the sound, and which manifested this power still more impressibly 

 later in the day, was now in front of us. On it the sonorous waves 

 impinged, and from it they were sent back with astonishing in- 

 tensity. The instruments, hidden from view, were on the summit 

 of the cliff 235 feet above us, the sea was smooth and clear of 

 ships, the atmosphere was without a cloud, and there was no object 

 in sight which could possibly produce the observed effect. From 

 the perfectly transparent air the echoes came, at first with a strength 

 apparently little less than that of the direct sound, and then dying 

 away." 



Professor Tyndall shows the necessity of using sound- 

 producing instruments in foggy weather instead of attempting 

 to penetrate the fog with a light : 



u The cloud produced by the puff of a locomotive can quench 

 the rays of the noonday sun ; it is not therefore surprising that 

 in dense fogs our most powerful coast lights, including even the 

 electric light, should become useless to the mariner. 



" Disastrous shipwrecks are the consequence. During the ten 

 years ending in 1874, no less than 273 vessels were reported as 

 totally lost on our own coast in fog or thick weather. The loss, I 

 believe, is far greater on the American seaboard." 



It is a curious and interesting fact that anything in the 

 nature of a light does best in perfectly clear weather, while 

 anything in the nature of a sound does best in thick or foggy 

 weather. Acoustic clouds that bar the passage of sound never 

 exist in foggy weather ; these clouds are produced by numerous 

 conflicting currents of air at different densities caused by the 

 interchange of the hot air from below with the cold air from 

 above, and this action never takes place in thick weather ; 

 therefore, acoustic clouds cannot interfere with the working 

 of my apparatus. 



xiv 



