voice in one direction so that they are kept from spreading widely, consequently by its use the voice may be heard several times the usual distance. The intensity of a sound is also affected by the _density_ of the transmitting medium. Thus a sound produced on a mountain top is fainter and thinner than one produced in a valley. The sound of a bell in the receiver of an air pump becomes weaker as the air is exhausted from the latter. _Four_ factors thus influence the intensity of a sound, the _area_ of the vibrating body, its _amplitude_ of vibration, the _distance_ of the source and the _density_ of the transmitting medium. It is well to fix in mind the precise effect of each of these factors. [Illustration: FIG. 323.--The megaphone.] =331. Pitch.=--The most characteristic difference between musical sounds is that of _pitch_. Some sounds have a high pitch, such as those produced by many insects and birds. Others have a low pitch as the notes of a bass drum or the sound of thunder. How notes of different pitch are produced may be shown by the siren (Fig. 324). This is a disc mounted so as to be rotated on an axis. Several rows of holes are drilled in it in concentric circles. The number of holes in successive rows increases from within outward. If when the siren is rapidly rotated air is blown through a tube against a row of holes a clear musical tone is heard. The tone is due to the succession of pulses in the air produced by the row of holes in the rotating disc alternately cutting off and permitting the air blast to pass through at very short intervals. If the blast is directed against a row of holes nearer the circumference the pitch is higher, if against a row nearer the center the pitch is lower. Or if the blast is sent against the same row of holes the pitch rises when the speed increases and lowers when the speed lessens. These facts indicate that the pitch of a tone is due to the number of pulses or vibrations that strike the ear each second; also that _the greater the rate of vibration, the higher the pitch_. [Illustration: FIG. 324.--A siren.] =332. The Major Scale.=--If a siren is made with eight rows of holes, it may indicate the relation between the notes of a _major scale_. To accomplish this, the number of holes in the successive rows should be 24, 27, 30, 32, 36, 40, 45, 48. If a disc so constructed is rapidly rotated at a uniform rate, a blast of air sent against all of the rows in succession produces the tones of the scale. These facts indicate that the relative vibration numbers of the notes of any _major scale_ have the same relation as the numbers 24, 27, 30, 32, 36, 40, 45, 48.