122 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK n. 



increased by placing the instrument on a table. It is wound up 

 by a key like a common watch. The cylinders are studded with pins 

 so that many airs can be played at will by moving the cylinder, 



FIG. 72. Musical-box. 



Figures 73, 74, and 75 represent instruments known under the 

 name of sistrums, and used in ancient Egypt. They were evidently 

 based on the vibrations of metal rods. The Jew's-harp, with which 



Fin. 73. Sistmni of Isis. 



FIGS. 74 and 75. Sistra of the ancient Egyptians. 



children still amuse themselves and which is probably of very great 

 antiquity, is an instrument which may be classed among these. It is 

 composed of a steel rod, free at one end and joined at the other to an 



