166 HT. A. Newton— Origin of Comets. 
whole number. We may therefore assume that comets to 
become visible to us ought, in general, to come nearer to the sun 
than that planet. All others may be regarded as permanently 
invisible. There is, however, no reason to doubt that many 
such unseen comets exist. Even those which we see become 
invisible at a moderate distance from the earth and sun. 
4. The orbits of most comets are so near toa parabolic form 
that it is only when they are very well observed that we can 
detect any deviation therefrom. They pass to a great distance 
from the sun, and it is reasonable to suppose that their origin, 
even on Kant’s hypothesis, was remote from the sun. e 
must interpret that hypothesis as meaning that some of the 
parcels of matter that would normally have gone to make up 
distant planets became scattered into comet masses. 
5. Consider such a parcel, or comet mass, A, at a point such 
that the line AS from A to the sun S is large; for example, 
1,000 times the distance from the earth tothe sun. Now if the 
ee would go around the sun at a great distance, and 
would belo 
point A, or rather shot from the point A. Through S draw @ 
plane perpenmeuly, to AS, aint n that plane draw a circle 
whose ius is twi i 
and circle we may regard as a target at which the several 
cometic masses may regarded as launch those 
whose velocities icular to AS small will strike 
within. the circle, and so coming nearer to the sun than Mars 
will form part of the group of comets which we know any- 
thing about. 
