258 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
observed fact. Smaller and less dense particles must travel more 
rapidly than the larger and denser ones. 
The constant expulsion of matter along the tail into outer space 
must of necessity cause a comet to grow smaller. Disintegration is 
continuous, and the tail at any moment is made up of materials lost 
forever from the nucleus. Several faint comets moving around the 
sun In small orbits have been observed to be fainter at each succes- 
sive return. Some have even disappeared entirely. Two such com- 
ets, now lost to view, reveal themselves. only by virtue of meteor 
showers about the middle of August and the middle of November; 
the matter composing their nuclei has been scattered along their 
orbits, and the annual passing of the earth across these orbits leads 
to collisions between the cometary fragments and our higher atmos- 
phere. There is no reason to doubt that Halley’s comet is slowly 
disintegrating, and, after long ages, will suffer some such fate. 
Our knowledge of the chemical composition of comets and of the 
‘state in which cometary matter exists is meager and unsatisfactory. 
A few give spectra very like that of our own sun, indicating that 
they are shining by reflected sunlight, as the planets shine. Other 
comets send out their own light, almost exclusively, the radiations 
coming chiefly from carbon and cyanogen sources. Still others have 
mixed spectra, showing both inherent hight and reflected light. Why 
comets shine by virtue of light within themselves is a mystery, for it 
is difficult to conceive that such attenuated bodies should have the 
heat of incandescence throughout their mass. Although many com- 
ets have volumes thousands of times as great as the sun’s volume, 
their total mass is insignificant even in comparison with that of the 
earth; and such mass as they have is nearly all in the nucleus. The 
tails are surely less dense than the most perfect vacuum we can 
produce in the laboratory. 
Halley’s comet is due to pass near the earth in May, 1910, perhaps 
within 10,000,000 miles of us. Let no one draw the inference that 
there may be a dangerous collision with the earth, for such is not the 
‘ase. Their paths are too widely separated. Even if the path of 
the comet were entirely unknown, we could say that the chance of a 
collision with the denser nucleus is so small as not to call for consid- 
eration. And if we should pass through the tail there would be no 
evidence of such an encounter, unless it consist of a harmless meteor 
shower, for the tails of comets are certainly composed of exceedingly 
minute and widely scattered particles. 
The ancients thought of comets as hairy objects, from the appear- 
ance of the tails; hence the origin of the term “comet,” from the 
Greek kometes, signifying “ long-haired.” This belief prevailed cer- 
tainly up to Halley’s day and generation. 
