CHAP. II.] 



SIMPLE INSTRUMENTS. 



133 



is thicker than the lower one, which vibrates under the influence of 

 the cylindrical mass of air inside. Two catguts are stretched across 

 the drum and placed against the skin ; in vibrating, they strike the 

 skin and give a peculiar tone to the note. 



lUir^^^s* 



Fro. 89. The Hing Kou. 



Drums can be made with notes which together form a musical 

 harmony of a third, fifth, or octave. For this they must be of homo- 

 logous dimensions in the inverse ratio of the numbers, 1, J, 4, 2, that 

 is to say, proportional, for instance, to the numbers 30, 24, 20, and 

 15. This is the law of the vibrations of the columns of air inclosed 



