METEORS AND COMETS. 
193 
November once every thirty years, and in August we 
cross the path of another exploded comet. 
Many of the meteors are not bright enough to be 
seen with the eye. From telescopic observations 
Arago estimated that in three hours over 243,000 
meteors fell. It is thought 600,000 tons of meteoric 
dust falls annually upon the earth. The well-known 
astronomer Simon Newcomb estimated that 146,- 
000,000,000 meteors reached the earth yearly. Thus 
we see that our globe is 
receiving material addi¬ 
tions from interstellar 
space. 
Although so many me¬ 
teors fall upon the earth, 
they very seldom do much 
harm. Instances, how¬ 
ever, are on record where 
a man has been killed or 
a house set on fire. 
As has been intimated, 
meteors are solid bodies 
and not gases. When 
they reach our atmosphere 
the friction makes them 
red-hot, so that they give off illuminated particles 
which mark their path. Some are of such structure 
that they are entirely consumed before they reach the 
earth. These are called shooting stars. Others, 
larger and more solid, are known as bolides, and often 
they explode with a dull thunder, and the remnants 
Fig. 96.—Famous comets. 
