102 WELLS'S NATURAL PHILOSOPHY. 



mitted by water near four times more rapidly than by air, 

 and by solids about twice as rapidly as by water. 



If wo strike two stoucs together under water, the sound will be as loud as 

 if they had been struck in the air. 



When a stick is lield between the teeth at one extremity, and the other is 

 placed in contact with a table, the scratcli of a pin on the table may be heard 

 ■with great distinctness, though both ears be stopped. 



The earth often conducts sound, so as to render it sensible to the car, when 

 the air fails to do so. It is well known that the approach of a troop of horse 

 can be heard at a distance by putting tlie ear to the ground, and savages 

 practice this method of ascertaining the approach of persons from a great dis- 

 tance. 



The principle that solids transmit sounds more perfectly than air, has been 

 applied to the constniction of an instrument called the "stethoscope." 

 ^ . The stethoscope consists of a hollow cylinder of wood, somo- 



Stetlioscope. what resembling in form a small trumpet. The wide mouth 

 is applied lirmly to the breast, and the other is held to the ear 

 of the medical examiner, wlio is thus enabled to hear distinctly the action of 

 the organs of respiration, and judge whether they are in a healthy condition, 

 or the reverse. 



How is the in- 413, Souud dccreascs in intensity from the 

 affbcted°b'''di'^- center where it originates, according to the 

 tance? same law by which the attraction of gravita- 



tion varies, viz., inversely as the square of the distance. 

 That is to say, at double the distance it is only one fourth 

 part as strong ; at three times the distance, one ninth, and 

 so on. 



This law applies with its full force only when no opposing currents of air, 

 or otlier obstacles, interfere witli the wave movements, or undulations. By 

 confining the sound undulations in tubes, whicli prevent their spreading, tho 

 force of sound diminislies mucli less rapidly. It will, therefore, under such 

 circumstances, extend to much greater distances. This ^jrinciple is taken ad- 

 vantage of in the construction of speaking-trumpets. 



Sounds can generally be heard, especially on a calm day, at 

 h^^niTnorT.u's- ^ greater distance upon water than upon land. The plane 

 tinotly upon surface of water, as a smooth wall, prevents the lateral spread- 

 Und f '^^ '^ ill? ^^d dispersion of the sound-waves, although on only one 

 side. The air over water, owing to the presence of moist- 

 ure, is also generally more dense, and the density more uniform than 

 over the land. Water, in addition, is a better conductor of sound than 

 the earth. 



Tho transmission of sound from one apartment to another may be prevented 

 by fining up the spaces between the partition walls with sliavings, or any 

 porous substances. The number of media tlu-ough which the sound must 



