Some Properties of Matter. Грекова О.А. - 17 стр.

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requires something material, a gas, a liquid, or a solid, to convey it. If there were a huge
explosion on the moon, we should, of course, see it, but we should hear nothing. Long ago
Robert Boyle proved that sound would not travel through empty space. He hung up a watch by
a fine thread in a glass vessel, and showed that, when the air was pumped out, no sound of
ticking could be heard, although it was plain before. He also arranged a bell so that it could be
struck in a vessel, from which the air had been removed, with the same result
               That sound travels well through a liquid can be easily proved by putting the head
under water in the sea, or in a river or lake. Men can hear distant horsemen by putting their ear
to the ground, an example of sound travelling through a solid. Sound that we normally hear is,
then, a longitudinal wave in air, which means that the particles are vibrating backwards and
forwards in the direction in which the wave is travelling.
                Sound travels very slowly compared to light the rate at which it passes through
air depends somewhat on the temperature, being greater for higher temperatures. Thus at 80 o F.
(26.7o C.) the velocity in dry air is 1141 feet per second; at freezing point, 32o F (0o C.), it is
1087 feet per second. One can judge the distance, in feet, of a thunderstorm, by noting the
seconds between the sight of the lightning and the first sound of the thunder, and multiplying
the number by 1100. Thus an interval of five seconds corresponds to a distance of 5500 feet,
about a mile. Sound travels through liquids much more rapidly than through air. It travels more
rapidly still through metals. The velocity through the earth naturally depends upon the nature
of the materials.
                Sound, like light and all wave motions, can be reflected, but for good reflection
of ordinary sound we need a flat surface several feet across, a blank wall or a flat cliff-face, for
instance. This is because the reflecting surface must be several wavelengths across if the
reflection is to be satisfactory. Any sharp sound, such as that of clapping the hands, made in
front of such a surface will be heard again after a short interval – the so called echo. The
interval is the distance there and back from the clapping listener to the reflecting surface,
divided by the velocity of sound, so that for an interval of 1 second, the listener, who is also
the source of sound, must be about 550 feet from the surface.
                Important practical use has been made of the reflection of sound in recent years.
At sea the so-called ‘sounding’ is used to find the depth of the sea at any point. A like method
has even been used for detecting and measuring the depth of a shoal of fish, the sound being
reflected from the immense numbers of fish. Sound reflection has also been used for the
detection of submarines. In all cases, sound of very high frequency, so high that it cannot be
heard – that is, of very short wavelength – is used, in order to get sharp reflection. Needless to
say, the modern apparatus used can detect and register the arrival of such waves very precisely.

                                     Comprehension check:
      1)   What is sound?
      2)   Can sound travel in completely empty space?
      3)   What experiment did Robert Boyler carry out to prove his idea?
      4)   Sound travels very slowly compared to the light, doesn’t it? How can we judge the
           distance of a thunderstorm?
      5)   What do we need for good reflection of ordinary sound?
      6)   What is echo?
      7)   How is the sound reflection used for more practical purposes?

     IV Writing:

   1. Give a short summery of the text.