THE STETHOSCOPE. 523 



sound being transmitted through wood in a most remarkable 

 manner. 



For example, if one end of the longest scaffolding pole be 

 slightly scratched with a pin, the sound will be distinctly- 

 heard by any one who places his ear against the other end, 

 though the person who uses the pin can scarcely hear the 

 sound himself. The surgeon, therefore, places the broad end 

 of the Stethoscope upon the patient, and the other upon his ear, 

 taps more or less lightly with his fingers, and by the sounds 

 transmitted through the Stethoscope ascertains the condition 

 of the internal organs. 



On the left hand is an illustration of the mode in which the 

 Australian savage, without the least idea of the theory of 

 Acoustics, utilises the sound-conducting power of wood. If 

 he wishes to know whether or not a hollow tree is tenanted by 

 an animal of which he is in pursuit, he places his ear against 

 the tree, taps it smartly with his tomahawk, and listens for the 

 movement of the animal inside. 



So delicate is this test, that it is employed even when the 

 native is hunting for the large beetle- grubs on which they 

 feed, and which are accounted a luxury even by Europeans, 

 when they have once overcome the prejudice attaching itself 

 to eating, without cookery, fat white grubs as thick and long 

 as a man's finger. 



The Aye- aye is said to eat in exactly the same manner, 

 tapping with its long finger the trunks and branches of trees, 

 and, if it hears a maggot inside, gnawing it out. 



Measurement of Sound. 



Of late years we have had an instrument which enables 

 us to measure the vibrations of sound as accurately as the 

 barometer measures the weight of the atmosphere, the thermo- 

 meter the temperature, and the photometer the power of light. 

 This is the Siren, which is shown on the right hand of the 

 accompanying illustration. 



To explain this instrument fully would require ten times 

 the space which we have at command, and necessitate a great 



