Page:The story of the comets.djvu/70

planet, be it the Earth or some other planet; and help in an infinitesimal degree to feed that planet with new material from an external source of supply.

It must be confessed that progress in the collection of facts up to this point has done little or nothing to settle the questions, "Why should any comet have a tail?" and "What is the nature of the Solar or other influence which causes tails?" Many have been the sober, and still more the ridiculous, suggestions which have been put forth on this subject, but it may safely be said, following Olbers, Bessel, and Sir J. Herschel, that electricity, operating in some unknown and indefinite way, is the primary agent in setting on foot all cometary tails, but as to why and how, there is no agreement amongst astronomers.

As an alternative to, or rather, a co-operating force with, electricity much support has been accorded to the idea that "Light-Pressure" is now and again (or always) to some extent concerned in the repulsive action of the Sun on the tails of comets. The subject of Light-Pressure is one which belongs rather to the domain of Physics than of Astronomy. I will therefore only say that it is supposed that all sources of light exercise a certain amount of repulsion, or push, on all material substances which face the source of light, whatever may be the material, or whatever the source of the light.

Maunder has summarised the questions both of the heads and of the tails of comets in a way which seems to represent all that we really know. He says:—

"Though the bulk of comets is huge, they contain extraordinarily little substance. Their heads must contain some solid matter, but it is probably in the form of a loose aggregation of stones enveloped in vaporous material. There is some reason to suppose that comets are apt to shed some of these stones as they travel along their paths, for the orbits of the meteors that cause some of our greatest 'star showers' are coincident with the paths of comets that have been observed. But it is not only by shedding its loose stones that a comet diminishes its bulk; it loses also through its tail. As the comet gets close to the Sun its head becomes heated, and throws off concentric envelopes, much of which consists of matter in an extremely fine state of division. Now it has been shown that the radiations of the Sun have the power of repelling matter, whilst the Sun itself attracts by its gravitational force. But there is a difference in the action of the 2 forces. The light-pressure varies with the surface of the particle upon which it is