Item of the Day: Oliver's Essay on Comets (1772)
Full Title:
An Essay on Comets, in Two Parts. Part I. Containing an Attempt to explain the Phænomena of the Tails of Comets, and to account for their perpetual Opposition to the Sun, upon philosophical Principles. Part II. Pointing out some important Ends for which these tails were probably designed: Wherein it is shewn, that, in Consequence of these curious Appendages, Comets may be inhabited Worlds, and even comfortable Habitations; notwithstanding the vast Excentricities of their Orbits. The Whole interspersed with Observations and Reflections on the Sun and Primary Planets. By Andrew Oliver, Jun. Esq.
Written by Andrew Oliver, Jr. Printed and sold in Salem, New-England by Samuel Hall, near the Exchange, 1772.
From Part II:
AS ancient geographers imagined the polar and equatorial regions, or the frigid and torrid zones of the earth, were uninhabitable, in consequence of the extremes of heat and cold, to which those climates are exposed: So, modern astronomers have passed a similar judgment upon the superior and inferior Planets, especially on Saturn and Mercury; concluding, that our water would always boil upon the latter, and be frozen upon the former; and that merely in consequence of their different distances from the Sun. Whence it has been naturally concluded, that the textures of their various fluids, and of their inhabitants, to whose uses these fluids are adapted, are very different from what they are found to be upon our Earth: And, considering the near approaches of most Comets to, and the vast elongations of all their orbits from the Sun, it has been generally supposed, that no material race of beings could subsist under such amazing vicissitudes of heat and cold, as those bodies must, from their different situations, necessarily be exposed to; consequently that they are uninhabited.
BUT the conclusiveness of this reasoning depends upon the truth of the following Proposition; advanced indeed by Sir Isaac Newton; but not supported by experiments, which were, with him, the criterion veritatis; viz. that, "The heat of the Sun is as the density of his rays, that is reciprocally as the squares of the distances from the Sun."
HERE, we are again reduced to the disagreeable necessity, of dissenting from the opinion of the greatest GENIUS that ever dignified human reason; which, considering the justly celebrated fame of that illustrious author, may be stigmatized as ignorance or vanity: But it is hoped that the reader will wave that imputation, if he shall judge, upon the whole, that Sir Isaac himself would have altered his opinion, upon the evidence which we shall produce in support of the contrary position: We may, however, lay down this as a maxim, that, in the prosecution of any science, the progress of the mind must necessarily be retarded, in proportion to the implicit assent we give to the decisions of any man, however great. We shall therefore, without further apology, endeavour to prove that the heat of the Sun, as perceived by us, and as discoverable by its effects upon other substances exposed to his rays, does not depend upon the density of those rays only, though they are necessary to the very existence of heat; but, equally upon the concurrent operation of another cause, which we shall presently consider; from whence it will follow, that these causes, wherever they coexist, whether upon the Earth, or upon the heavenly bodies, will naturally produce similar effects.
IN the mean time, before we engage in the discussion of planetary heat, as depending upon the several distances of the Planets from the Sun; it may throw some light upon this subject if we consider the portion of that heat which falls to our own share, and the distribution of it throughout the various climates of the Earth. [...]
AS a Comet approaches its perihelion, that hemisphere of its atmosphere which is next to the Sun, being more immediately exposed to his rays, will feel the effects of his neighbourhood sooner than the opposite hemisphere, and consequently will be warmed, rarefied, and thrown off behind the Comet by the repulsion of the Sun's atmosphere, sooner than the other; the colder and denser parts of the fluid will of course continually flow in from the other side of the Comet to supply its place, in order to preserve, as near as may be, an equilibrium; in consequence of which there will be a constant succession of the cooler air from thence; whereby the inhabitants on the hemisphere next the Sun may be continually refreshed with gales of wind during that vicinity, which would increase till the Comet arrived at its perihelion, when their velocity would be greatest of all; but even then they would not (from this cause) blow in sudden violent gusts like our hurricanes, but steadily, unless disturbed by causes from within the Comet's atmosphere; besides, as the velocity of the current increased, the density of the fluid would lessen from the increasing rarefaction, whereby its momentum might continue nearly the same; for this momentum would be in a ration compounded of the velocity of the fluid and its density together; and as the violence of our high winds, and their consequent effects depend, not upon the velocity, merely, but upon the momentum of the current, this brisk circulation of the cometic air may, (however great we suppose its velocity) be rather grateful than injurious to the Cometarians: And how unfit their use, if stagnant, yet, when thus put in motion, it may be rendered sufficiently active to answer all the purposes of respiration. This reasoning is confirmed by daily experience: For it is not an uncommon thing for people of tender frames to faint in a close hot and rarefied air; and as the fan is generally near at hand, it is as common for the by-standers to apply it to their faces, which, by giving a brisk motion to the air, without any alteration of its density, generally revives them, in a short time, even when no other remedy is at hand. --- This brisk motion of the air would also remove or prevent the disagreeable sensations of heat which the cometary inhabitants might otherwise suffer from an exposure to the Sun's rays at their perihelia: For, if a person sit with his face uncovered before the scorching blaze of a common fire, the motion of the air excited by a common fan, even without hiding the blaze from the face, is suggicient, not only to make the situation comfortable, but to change the painful sensation to an agreaable coolness: As any one will find upon trial.
