NAVIGATION. 123 



the 1874 Report of the United States Lighthouse 

 Board, in official papers printed by the House 

 of Commons in 1874 and 1875, and in the recent 

 edition of Tyndall's Lectures on Sound, very 

 interesting and important results of observations 

 are described, showing that in certain states of 

 the atmosphere (which seem to depend on a 

 streaky distribution of density, due to the com- 

 mingling of warmer and colder air, or as suggested 

 by Professor Osborne Reynolds, on an upward 

 curvature of the lines of propagation of sound 

 due to colder air above than below, or on both 

 causes combined) sound ceases to be heard at 

 extraordinarily small distances. One thing 

 brought out by these investigations is, that a 

 fog, however dense, is by no means unfavourable 

 to the transmission of sound, and that it is often 

 in clear bright days that sound travels worst. 



65. In respect to navigation, it is satisfactory to 

 know that in the densest fog, with moderate weather 

 (and dense fogs generally occur only in moderate 

 weather), a sailing ship or steamer, sufficiently and 

 judiciously using a fog-horn, such as the most 



