190 THE HUMAN BODY. 



velocity is not changed if the wind blows perpendicularly to 

 this direction. Sound cannot be produced in a vacuum, and 

 it is therefore less intense in proportion as the air is more 

 rarefied. It is weaker, for instance, on the tops of high 

 mountains than in the lower strata of the atmosphere, 

 although the profound silence which reigns at times in these 

 elevated regions permits even very feeble sounds to be 

 heard at great distances. We were enabled to prove this 

 with M. Martins in 1844. Near St. Cheron (Seine-et-Oise), 

 at an elevation of 459 feet, a diapason placed on a drum 

 could be heard in the daytime 277 yards off; while on the 

 great plateau of Mont Blanc, at a height of 13,123 feet, 

 the sound of the same instrument could be heard at a 

 distance of 368 yards. On the top of Mont Blanc, we 

 could hear our guides talk at a distance of 437 yards, and 

 they could hear us speak also. 



Humboldt observes that sound is more intense and is 

 propagated farther in the night than in the daytime, in spite 

 of the noises and of the wind which in tropical countries 

 increase after sunset. This diminution in sounds during the 

 day is attributed by the illustrious observer to the unequal 

 temperature of the strata of the atmosphere, under the influ- 

 ence of the sun and the radiation from the earth. 



Sound moves much more quickly in water and in solid 

 bodies than in the air. Colladon and Sturm found its 

 velocity to be 4708 feet in a second in the waters of the 

 Lake of Geneva at 8 C. (46*4 F.) of temperature; according 

 to the experiments of Biot, its average velocity is 10,663 ^ eet 

 in cast-iron pipes. This is about five times greater in water 

 than in air, and nine times greater in the pipe. 



Humboldt records that sometimes volcanic detonations 

 have been transmitted through the earth a distance of 500 to 

 745 miles. 



It is stated that the gravest sound which can be perceived 

 by the ear is 32 vibrations in a second (16 according to 

 Savart), and the most acute, according to Despretz, is 73,700 

 vibrations. A sound of 60,000 vibrations is, according to 

 M. Martins, very feeble, difficult to hear, and of such sharp- 

 ness as to cause a painful impression on the ear. The 



