PROGRESS OF METEORIC ASTRONOMY IN AMERICA. 303 
and meteorites differ in magnitude and appear under widely 
varying conditions, but from an astronomical standpoint they 
are all alike. 
They are all solid bodies and are fragments of comets. 
Assuming that this theory is true, we shall find that some 
of the inferences drawn from it are of great importance in 
their bearing on cosmical physics. 
1st. As the meteoric masses, both great and small, are 
derived from comets, they must have originated beyond the 
limits of the solar system, and they furnish evidence of the 
existence in space of exactly such minerals, though in dif¬ 
ferent combinations, as are found in the earth’s crust. 
2d. They arise from the disintegration of comets, which 
for centuries have furnished the material for the enormous 
areas of bodies forming the various meteor streams that trail 
along the orbits of these masses for immense distances. 
3d. The meteors forming the shower of November 13-14 
have been observed for more than 900 years, and yet the 
comet whose gradual destruction has produced these bodies 
was not discovered until 1866. The August meteors have 
been observed for more than six centuries, but the comet 
whose disintegration has furnished the material for this vast 
stream remains intact, and was not discovered until 1862. 
The accepted comet-meteor theory does not explain clearly 
the visibility of comets or the changes that occur in the ap¬ 
parent brightness and in the density of the nucleus as these 
bodies approach and recede from the sun; neither does it 
explain in a satisfactory manner the position of the comets 
in their attendant meteor streams. If comets are composed 
of solid matter or of discrete solid particles, it would seem 
quite proper to ask why they become visible at such im¬ 
mense distances from the earth and the sun. 
The perihelion distance of 26 per cent, of the comets with 
known orbits is equal to or greater than the mean distance 
of the earth from the sun. 
Many comets when first seen are much farther from the 
sun than is the earth at aphelion, and the spectroscope only 
