THE ATMOSPHERE AND FOG-SIGNALING. 541 



not be the only effort attempted for the purpose of making prac- 

 tical astronomy flourish among us with a splendor worthy of a 

 glorious past. 



THE ATMOSPHERE IN RELATION TO FOG-SIGNALING. 



By JOHN TYNDALL, LL. D., F. E. S. 

 INTRODUCTION. 



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 last ten 

 years no less than 273 vessels have been reported as totally lost on 

 our own coasts in focc or thick weather. The loss, I believe, has been 

 far greater on the American seaboard, where trade is more eager and 

 fogs more frequent than they are here. No wonder, then, that earnest 

 efforts should be made to find a substitute for light in sound-signals, 

 powerful enough to give warning and guidance to mariners while still 

 at a safe distance from the shore. 



Such signals have been established to some extent upon our own 

 coasts, and to a still greater extent along the coasts of Canada and 

 the United States. But the evidence as to their value and perform- 

 ance is of the most conflicting character, and no investigation suffi- 

 ciently thorough to clear up the uncertainty has hitherto been made. 

 In fact, while the velocity of sound has formed the subject of refined 

 and repeated experiment by the ablest philosophers, the publication 

 of Dr. Derham's celebrated paper in the " Philosophical Transactions " 

 for 1708 marks the latest systematic inquiry into the causes which affect 

 the intensity of sound in the atmosphere. 



Jointly with the Elder Brethren of the Trinity House, and as their 

 scientific adviser, I have recently had the honor of conducting an in- 

 quiry designed to fill the blank here indicated. 



One or two brief references will suffice to show the state of the 

 question when this investigation began. " Derham," says Sir John 

 Herschel, " found that fogs and falling rain, but more especially snow, 

 tend powerfully to obstruct the propagation of sound, and that the 

 same, effect was produced by a coating of fresh-fallen snow on the 

 ground, though when glazed and hardened at the surface by freezing 

 it had no such influence." 1 



In a very clear and able letter addressed to the President of the 

 Board of Trade in 1863, 2 Dr. Robinson, of Armagh, thus summarizes 

 our knowledge of fog-signals : 



1 "Essay on Sound," par. 21. 2 " Report of the British Association " for 1863, p. 105. 



