Proceedings of the American Philosophical Society Held at Philadelphia for Promoting Useful Knowledge Volume 3 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1843 edition. Excerpt: ...remains the same, revolving along with the nucleus until more matter chances to be driven off to add to its brightness and extent; which has been the received notion hitherto; or--2. By continually urging a portion of the cometic matter away from the head an indefinite distance into free space; in which case the tail, as seen by us at any instant, would be but the collection of all the particles that had been emitted from the head during a certain previous interval, viewed in the act of darting off into free space. This theory, so far as known, has not been propounded before by any astronomical writer. Agreeably to this notion, the tail increases in length towards the perihelion, by reason of a more copious evolution of shining nebulous matter, in consequence of the increased heat of the sun. It diminishes in brightness in receding from the head, and at last becomes too faint to be discerned, from the following causes: --1. A more rapid flow of the matter by reason of a longer continued action of the sun's repulsive force. 2. An increase in the breadth of the tail. This may be supposed to arise from the divergence of the lines of direction of the forces acting upon the outer parts of the envelope, and, in some cases also, from a rotation of the tail about its axis, generating a centrifugal force. The tails of some comets are known, from observation, to have had a motion of rotation about their axis, as the comets of 1769 and 1825, and Halley's comet at its last appearance. 3. An augmentation in the distance of the matter from the sun, the supposed source of its light. The following objections were then urged against the received theory: --1. No good reason can be assigned why the sun's repulsive force, so called, should not drive off the...