56 PRACTICAL LESSONS IN SCIENCE. 



dangerous places during fogs and thick weather, which make the 

 sense of sight of no value. All the maritime nations have been 

 experimenting for years on various kinds of sound signals. These 

 experiments have resulted in some interesting facts about the air 

 and some of its peculiarities in transmitting sound waves. Prof. 

 Tyndall found the capacity of the air to transmit sound waves 

 vary so much that no laws could be discovered. He experimented 

 with guns, fog horns, steam whistles, and a steam syren. He 

 found that frequently, under a clear sky, when the air seemed 

 especially transparent to the sense of sight, it was especially 

 opaque to sound waves. These acoustic clouds so filled the 

 transparent air, that guns, fog horns, whistles and syrens which 

 had been heard from 10 to 13 miles, could not be heard at a dis- 

 tance of three miles. The interrupted sound waves seemed to be 

 wasted by repeated reflections from streams of air differently 

 heated or saturated in different degrees with water vapor. 

 Neither rain, hail, snow, or fog seem to have any power to ob- 

 struct sound waves. In fact, air that is filled with water vapor, 

 in some form, is in a highly favorable state for the transmission 

 of soundwaves. Sometimes the guns could be heard moreclearly, 

 sometiines the whistles, and sometimes the syrens; sometimes 

 low pitched, long-waved sounds could be heard more distinctly, and 

 again the high tones penetrated farthest. Prof. Tyndall thought 

 the steam syren the best instrument, considering it reliable for 

 two miles, and generally for three and three and one-half miles. 

 In general, when it could not be heard at least four miles, the 

 sky would be optically clear so that the sound signal would not 

 be as necessary as in thick weather. Other things being equal, 

 sound waves travel better with the wind than against it. The 

 velocity of sound in water is given by some authorities as about 

 4,700 ft. per second; in caoutchouc, as 197 ft. per second; in 

 lead, as 4,030 ft.; in copper, as 11,666 ft.; in oak timber, along 

 the grain or fiber, 12,622 ft.; in iron, as 16,800 ft. per second. *In 

 wet sand, as 825 ft. ; and in solid granite, as 1,664 ft. in a second. 

 Sound waves are often reflected from plane or curved surfaces, 

 causing echoes. At Woodstock, England, an echo returns 14 



