or lessening
the tension of the head by means of thumb-screws which act on a metal
ring.

[Illustration: KETTLE-DRUMS. Diameter of Head, 24-1/2 in. and 27-1/2
in.]

The other important members of the percussion family are shown on this
and the following page, their use being so obvious as to require no
detailed explanation.

[Illustration: TAMBOURINE. Diameter, 10 to 12 in.]

[Illustration: BELLS. (Fr. carillon; Ger. Glockenspiel.)]

[Illustration: SIDE DRUM. Diameter, about 15-1/2 in.]

[Illustration: TRIANGLE. Height, about 8 in.]

28. The _harp_ is one of the oldest of instruments (dating back over
6000 years), but it is only in comparatively recent years that it has
been used in the symphony orchestra. Its range is from [Illustration:
CC-flat] to [Illustration: f-flat'''].

[Illustration: HARP. Height, 5 ft. 8 in.]

The modern _double-action harp_ has forty-six strings, which are tuned
in half-steps and whole-steps so as to sound the scale of C[flat] major.
It has a series of seven pedals around its base, each pedal having two
_notches_ below it, into either of which the pedal may be lowered and
held fast. The first pedal shortens the F[flat] string so that it now
sounds F, (giving the key of G[flat]); the second one shortens the
C[flat] string so that it sounds C (giving the key of D[flat]); the
third pedal shortens the G[flat] string so that it sounds G (giving the
key of A[flat]); the fourth changes D[flat] to D (giving the key of
E[flat]), and so on until, when all the pedals are fixed in their first
notches, the scale of C is sounded instead of C[flat] as was the case
before any of the pedals were depressed. But if the first pedal is now
pushed down into the second notch the original F[flat] string is still
further shortened and now sounds the pitch F[sharp] (giving us the key
of G), and if all the other pedals are likewise successively lowered to
the second notch we get in turn all the _sharp keys_--D, A, E, B,
F[sharp] and C[sharp], the last-named key being obtained as the result
of having all the pedals fixed in their second notches, thus making all
the tones of the original C[flat] scale a whole-step higher so that they
now sound the C[sharp] scale.

Chords of not more than four tones for each hand may be played
simultaneously on the harp, but arpeggio and scale passages are the
rule, and are more successful than simultaneous chords. The notation of
harp music is essentially like that of piano