ity:--(1) instruments producing
sounds of definite musical pitch, and qualified thereby to take part in
the harmony of the orchestra, such as the kettledrum (q.v.); (2)
instruments of indefinite sonorousness, and therefore excluded from the
harmony of the orchestra; such are the bass drum, the side or snare
drum, the tenor drum, the tambourine, all used for marking the rhythm
and adding tone colour.

Drums are further divided into three classes according to special
features of construction:--(1) instruments having a skin stretched over
one end of the resonant cavity, the other being open, such as the
tambourine (q.v.) and the _darabukkeh_ or Egyptian drum, shaped like a
mushroom; (2) instruments consisting of a cup-shaped receptacle of
metal, wood or earthenware entirely closed by a skin or vellum stretched
across the opening, as in the kettledrum; (3) a receptacle in the shape
of a cylinder closed at both ends by skins, as in the bass drum, side
drum, &c.

Skin or parchment only acquires the elasticity requisite to produce
vibration by tension; the vibrations of the parchment are taken up by
the air enclosed in the receptacle, which thus reinforces the sound
produced by the parchment. The _tone_ of the instrument whether definite
or indefinite depends upon the dimensions of the vellum, the shape of
the resonant receptacle, and the method of percussion. The _intensity_
of the sound depends upon the degree of percussive force used and the
diameter of the vellum in proportion to the dimensions of the resonant
receptacle; the material of which the latter consists has little or no
influence on the tone of the instrument. The _pitch_ of the sound is
determined by the dimensions of the vellum taken in conjunction with the
degree of tension, the pitch varying in acuteness directly with the
degree of tension and inversely with the size of the vellum.

[Illustration: FIG. 1.--Military Bass Drum (Besson & Co.)]

The _bass drum_ or Turkish drum (Fr. _grosse caisse_; Ger. _Grosse
Trommel_; Ital. _gran cassa_ or _tamburo grande_) consists of a short
cylinder of very wide diameter covered at both ends by vellum stretched
over thin hoops, which in turn are kept in place by larger hoops fitting
tightly over them. At regular intervals in the two large hoops are bored
holes through which passes an endless cord stretched in zig-zag round
the cylinder and connecting the two hoops. The tension of the vellum is
controlled by means of lea