Now I do not think our sun would necessarily suffer very much from any of these known comets. They may long since have parted with the greater quantity of their substance. But it is quite possible that even one of those well-known comets of the solar system might cause very serious outbursts of solar heat and light; and it is certainly not only possible but extremely probable that other comets, such as have visited the solar system on paths fortunately not bringing them near to the sun, would have worked much mischief had their paths been differently situated.
We know that Newton held this opinion. He considered the real danger from comets to reside, not in the possibility that one might strike our earth, but in the possibility that one, falling upon the sun, might excite that orb to a degree of heat so intense that all life on this earth would be destroyed. It is true that, in Newton's time, physical laws were not so well understood as at present, and a considerable portion of Newton's reasoning was consequently inexact. But nothing which is now known opposes itself to the belief which Newton adopted on this subject. On the contrary, whereas Newton only recognised the danger arising from the consumption of a comet as fuel for the sun, we now recognise a far more serious danger, from the force of meteoric impact, and the heat excited as the thermal equivalent of the destroyed velocities. Of this part of the danger Newton had no clear conception, the relations between mechanical energy and heat not having been established until quite recent times.
It appears to me, however, that the danger in the case of our own sun--or may we not say _our_ danger?--arises only from the possibility that some one of the comets which visit us from the star-depths may make straight for the sun; and this danger is exceedingly small. Almost certainly a comet which, leaving the domain of another sun, falls under the attractive influence of our own, would approach him on a path passing many millions of miles from his surface. The chances against a more direct approach are so great that they may be regarded as, to all intents and purposes, overwhelming. A comet _might_ visit us from the star-depth on a destructive course, just as a single black ball _might_ be drawn at the first trial from a bag containing a million white balls and only that single black one. But the danger is exceedingly small.