their Appearance, Nature, and Origin. 5 
The head of the comet of 1811 at one time measured 1,200,000 
miles, and its volume was about three times that of the sun. 
(B) The Nucleus.—This is the name given to a bright star-like 
point near the centre of the head. Its actual diameter ranges 
from 8000 miles to 100 miles. 
The nucleus appears to be the seat of considerable activity. 
Its dimensions are continually changing, and it throws out 
occasionally bright prominences which are called jets. It is 
often surrounded by bright concentric rings called envelopes. 
Fig. 3 (Plate) shows a picture of the comet 1901 (I.), observed 
at the Cape Observatory. The black streak behind the nucleus 
should be noted. The comet seems to leave a kind of double 
wake in space, shedding luminous matter on either side. This 
brings us to— 
(C) The Tail_—This is by far the most characteristic part. 
Its length varies considerably, but has been known to reach 
100,000,000 miles. The volume of the tail of the great comet 
of 1882 was more than 8000 times that of the sun. 
That the matter composing the tail is exceedingly attenuated, 
follows from the fact (slide shown) that stars shine through the 
tail of a comet with undiminished brilliance. 
But we have other reasons to believe that comets are extremely 
light bodies. We have seen how easily they are influenced by 
the attraction of Jupiter. The comets themselves, however, 
produce on Jupiter no appreciable effect. Thus Brook’s comet 
of 1889 actually passed inside the orbit of the nearest satellite of 
Jupiter. Its own path was completely changed; yet the dis- 
_ turbing effect on the planet’s path was practically nil. 
Now it is a well-known fact, that if two bodies react on one 
another, the effects produced in each are inversely as the mass. 
Thus, if a cannon be fired, the velocity of recoil is to the velocity 
of the shot in the inverse ratio of the mass of the cannon to that 
of the shot. If the velocity of the bullet be given, then the 
lighter the bullet, the less the recoil. In the same way with 
Jupiter, the lighter the comet, the less its disturbing effect. 
Calculation from data of this kind has shown that the mass 
of comets is certainly less than one 100,000th of the Harth’s 
mass. 
We have then to suppose the matter of comets to be made up 
either of very attenuated gas, or of very finely scattered dust, 
‘‘ pin’s heads several hundred feet apart,’’ as one authority has 
described it. 
Origin and Nature of Tail.—The appearance of a comet’s tail 
suggests, at first sight, a trail of matter left behind by the head 
in its course, like the smoke from a locomotive. But this 
appearance is deceptive; for, if this were so, the tail would 
always lie in the wake of the comet. On the contrary, the tail of 
