Daily National Intelligencer

High and sublime matters of consideration, relative to Planets and Comets—in a letter from John Harwood to Samuel L. Mitchill, dated Westchester, Pennsylvania, December 8, 1819.

Sir: I have read your letter to Mr Symmes, and am glad to see, that, without committing yourself by an assent to his theory, you yet encourage him to persevere in his enquiries.

A thought has occurred to me, in relation to the subject, which I shall take the liberty to suggest to you without apology.

The existing theory of the tails of comets is liable to insuperable objections. That they should possess an atmosphere which could be thrown off and extended in a stream to light many millions of miles, seems scarcely to deserve a serious refutation. Such a notion could only have been adopted in the absence of all probable causes to produce the phenomena.

The theory which I have the honor to submit to you results from the idea of Mr. Symmes, and is this:

That the comets are hollow bodies—that in their revolutions round the sun, they always present the hollow to that luminary—that the tail is caused by the sun's rays passing through the comet and extending within the cylinder of its shadow into the immense regions of space.

That the planets cast their shadows to immense distances, and that those shadows are quite dark, we know from experience. At the great eclipse in 1806, when the shadow of the moon fell upon the earth, I well recollect that the dew fell, the stars became visible, the fowls retired to roost, & night seemed to assert a settled though momentary reign.

Admit that a comet is hollow, and presents one of its openings to the sun; there would then be no doubt that the outside or solid part would throw its shadow, a cylinder of shade, into the most distant regions. It would be equally clear, that the centre of the shadow would be illuminated by the sun's rays passing through the hollow of the comet.

The question then is—Would the circumstance that the sun's rays were encircled by a body of shadow render those rays visible?

To settle this point I tried the following experiment. I took a tube 7 inches long and 2 in diameter: This I placed in a window so that the sun's rays would pass through it into a room, which I rendered otherwise dark. The result was, that a stream of light, like a rod of luminous matter, passed through the darkness.

My intention is now rather to hint the theory than to argue it at length. The phenomena of the extent, form, and situation of the comet's tail, would all be accounted for satisfactorily upon my hypothesis:

That it will not be rejected without a liberal examination, I am assured, from your character as a lover of science, a bold enquirer into the secrets of nature.

My principles of philosophical enquiry are, that we should be daring and adventurous in suggesting theories—patient and laborious in investigating them—cautious in adopting them.

With the greatest respect, I am yours,

JOHN HARWOOD.

Dr. SAMUEL L. MITCHILL.