132 



THE APPLICATIONS OF PHYSICAL FORCES. [BOOK n. 



The military drum is composed of a brass or wooden cylinder, 

 covered at the two ends with two skins stretched out by hoops, 



FIG. 87. Orchestral kettle-drums. 



FIG. 88. Persian drums. 



which may be tightened more or less by a system of cords outside 

 the cylinder. The upper skin, which is struck with the drumstick, 



