WHAT IS A COMET?

Otago Daily Times, Issue 13544, 17 March 1906, Page 8

WHAT IS A COMET?

An article under the above title, by Professor William H. Pickering, of Harvard University, in Harper's Magazine for February, tolls what the scientific world of to-day knows and believes about comets.

The most puzzling thing about a comet is its tail, which is always turned away from the sun. A comet approaches the 6un head first, and departs tail first. "According to the modern theory of electricity," writes Mr Pickering, "the sun is a negatively charged nody, from, whose surface vast numbers of minute bodies called corpuscles are being constantly repolled, at velocities not far from 100,000 miles per second. These corpuscles, electrons, or ions, as they are sometimes called, are much smaller than atoms, and constitute what was formerly rather crudely called negative electricity. Corpuscles that strike the comet immediately attach themselves to tho gaseous molecules surrounding the head, charging them ■negatively, and causing them to be repelled at high speed, not only from the other molecules forming the head, but particularly from the direction of tho negatively charged sun. The successive envelopes sometimes seen surrounding tho nucleus of a comet may indicate either some special activity going on within the comet itself, or they may indicate the effect of successive waves of corpuscles shot out from tho sun.

"The question may now naturally bo asked, Since the earth as well as the comet is surrounded by a gaseous envelope, why is not the earth itself also provided with a coraetary tail? We reply that at certain times it is. The great auroras that sometimes envelop both of the earth's polar regions, winding their wavering beams occasionally to an altitude of five or six hundred miles, aro .nothing else than a short cemetery tail to' our planet; the reason that a still greater length is not reaohed being simply on account of the enormous mas 6 of our earth, which will not permit any considerable portion of our atmosphere to escape far from it. "Since the gaseous particles receding from the head to form the tail can never again return to it, and since a comet owes its brilliancy chiefly to the electric illumination of these particles, it is obvious that a comet must become fainter and fainter' at every successive return ,to the sun. Moreover, the meteors forming the head, owing to their small gravitating force, which is further diminished by the positive electric charge left upon them by the recession of the tail, mil gradually separate from one another and distribute themselves uniformly along their orbit. Every cornet must therefore gradually disintegrate, the process taking place most, rapjdly in the case of comets having short periods or passing very near the bud."