=The Percussion Instruments= are the instruments of rhythm. In this class are included all instruments used for this purpose. It is universally admitted that rhythm is the very basis of music, without which it is vague and meaningless. Possibly the physical fact that lies behind rhythm is the tendency of all repeated muscular action to become regular; witness the blows of the hammer on the anvil, or the carpenter driving in a nail. The psychologic reason is that when the will has set a certain muscular action in motion, it leaves the carrying out of the command to some subordinate function, so _long as it is continuous_; but if the continuity is to be interrupted, the will must again exert itself; hence, drumbeating and rattle shaking must of necessity be rhythmic. Nearly all savages have dances of various kinds. Varieties of drum rhythms arise from the almost universal custom of accompanying dances with drums and rattles. =Varieties of Percussion Instruments=.—Percussion instruments are almost endless in variety. The most primitive example is that of a hollow log beaten with a war-club by some prehistoric savage. The next step leads to the hollow gourd or other hollow body, across the open end of which is stretched the dried membrane of some wild animal. From these descend all the long line of drums of all sorts, ending with the modern orchestral kettle-drums (tympani) which, by means of a mechanism for changing the tension of their parchment heads, may be tuned in various keys. Percussion instruments of metal are of very ancient origin. In this category are included cymbals of various sizes and shapes, gongs of all sorts, and later, bells and triangles. Comparatively few of the percussion instruments emit sounds of any definite pitch. They were and are to a great extent noise-producing, used for the purpose of marking rhythms.