152 
THE FAIRY-LAND OF SCIENCE. 
ear of the Alpine traveller as the glacier cracks on its 
way down the valley; or the mighty boom of the ava- 
lanche as the snow slides in huge masses off the side 
of the lofty mountain. Each and all of these create 
their sound-waves, large or small, loud or feeble, which 
make their way to our ear, and become converted 
into sound. 
We have, however, only time now just to glance at 
life-sounds, of which there are so many around us. 
Do you know why we hear a buzzing, as the gnat, the 
bee, or the cockchafer fly past? Not by the beating 
of their wings against the air, as many people imagine, 
and as is really the case with humming birds, but by 
the scraping of the under-part of their hard wings 
against the edges of their hind-legs, which are toothed 
like a saw. The more rapidly their wings move the 
stronger the grating sound becomes, and you will now 
see why in hot, thirsty weather the buzzing of the gnat 
is so loud, for the more thirsty and the more eager he 
becomes, the wilder his movements will be. 
Some insects, like the drone-fly (Eristalis te-nax), 
force the air through the tiny air-passages in their 
sides, and as these passages are closed by little plates, 
the plates vibrate to and fro and make sound-waves. 
Again, what are those curious sounds you may hear 
sometimes if you rest your head on a trunk in the 
forest? They are made by the timber-boring beetles, 
which saw the wood with their jaws and make a noise 
in the world, even though they have no voice. 
All these life-sounds are made by creatures which 
do not sing or speak; but the sweetest sounds of all 
in the woods are the voices of the birds. All voice- 
