33

motion was caused, the same impulse must cause a translation of his center: but then this hypothesis of the cause of the sun's rotation by impulse, is a mere hypothesis resting on no other ground than that of slight probability, and the hypothesis is still weaker with respect to the motion of those stars that change their magnitudes periodically; for we must first suppose that these changes arise from their rotatory motion, and secondly that their rotatory motion is a sign or symptom of their motion of translation.

But recourse is had to considerations of greater weight than theoretical considerations. If a star appears to have a motion, such motion may either be a parallactic motion, that is, caused by the motion of the solar system, or a real motion, or its motion may be compounded of a real and of a parallactic motion, in which case it will be represented by the diagonal of a parallelogram, of which the two sides are the above mentioned motions. But how shall we distinguish parallactic motions from real motions? Dr. Herschell says by their directions; for if a real solar motion exists, all parallactic motions will tend to a point in opposition to the direction of that motion; whereas all real motions will be in. discriminately scattered in space.

With these distinctions in view, (says, Dr. H.) we may examine the proper motions of the principal stars; for these, if the sun is not at rest, must either be entirely parallactic, or at least composed of real and parallactic motions; in the latter case they will fall under the denomination of one of the three motions we have defined, namely, sa, the apparent motion of the star.

In consequence of this principle, I have delineated the meet ing of the arches arising from a calculation of the 36 stars in Dr. Maskelyne's catalogue, on a celestial globe, and, as all great circles of a sphere intersect each other in two opposite points, it will he' necessary to distinguish them both; for if the sun moves to one of them, it may be called the apex of its motion, and as the stars will then have a parallactic motion to the opposite one, the appellation of a parallactic center may very properly be given to it. The latter falling into the southern hemisphere, among constellations not visible to us, I shall only mention their opposite intersections, and of these I find no less than ten that are made by stars of the first magnitude, in a very limited part of the heavens, about the constellation of Hercules. Upon all the remaining surface of the same globe, there is not the least appearance of any other than' promiscuous situation of intersections; and of these only a single one is made by arches of principal stars.'

The author then gives a short table of the ten intersecting points made by the brightest stars, which strongly indicates the parallactic effect which he is desirous of ascertaining: the proper motions of other stars are, however, examined, and the

argument for the parallactic motion strengthened. The object of the learned astronomer is to establish the reasonableness of the hypothesis of a solar motion, by making such solar motion explain in a great degree the observed proper motions of stars; that is, by resolving such proper motions either into mere parallactic ones, or into motions compounded of parallactic and very small proper motions.

The next object of Dr. H. is to establish the direction of, the solar motion: he takes two stars, Sirius and Arcturus, and from their proper motions in right ascension and north polar distance he calculates the arches in which such proper motions may be supposed to take place; these arches continued, meet in a point, to be called their parallactic center; the opposite point to this, is the required apex of the solar motion.

Having found out the apex, he takes the velocity of the sun to be such, that to a person situated at a star 90 degrees distant from the apex of the solar motion, and at a distance from the sun equal 1, the sun shall appear annually to describe an arch",84825; from such an hypothesis, and by the aid of a formula, the parallactic motions of Arcturus and Sirius are calculated, and these are found to agree with the proper motions established by observation.

of

Are all proper motions then parallactic, and is the point, the apex of the solar motion, really that towards which, the sun and his system is moving? Were this the case, then taking a third star, finding the arch in which it moves, and its intersection with the curve of the proper motion of one the preceding stars, such intersection ought to give the same solar apex as has been already determined. But if a third star, be taken, and the intersection determined, the apex of solar motion is not the same, and consequently the motion of this third star is not parallaetic solely, but is compounded of a real and of a parallactic motion. But if this third star (Capella) have a proper motion, why should not Sirius and Arcturus? And if they have, is not the former determination of the apex of solar motion erroneous ? Dr. H. thus takes notice of this objection:

This objection is perfectly well founded, and I have given the above calculation on purpose to shew that, when we are in search of an apex for the solar motion, it ought to be so fixed upon as to be equally favourable to every star which is proper for directing our choice. Hence a problem will arise, in our present case, how to and a point whose situation among three given apices shall be so that, if the sun's motion be directed towards it, there may be taken away the greatest quantity of proper motion possible from the given three stars. The intricacy of the problem is greater than at first it may appear, because by a change of the distance of the apex from any one of the stars, its parallacue motion, which is as the sine of

that distance, will be affected so that it is not the mere alteration, of the angle of direction, which is concerned. However, it will not be necessary to enter into a solution of the problem; for it must be very evident that a much more complex one would immediately succeed it, since three stars would certainly not be sufficient to direct us in our present endeavour to find the best situation of an apex for the solar motion; I shall therefore now leave these stars, and the apices pointed out by them, in order to proceed to a more general view of the subject."

The remaining part of the paper is employed in approx. imating to a point the apex of the solar motion, which shall be so situated as to give to the proper motions of the fixed stars the least quantity possible. We have thus, somewhat in detail, and fully, stated the arguments and investigation of the ingenious astronomer, rather from respect to his fame and talents than from conviction either of the accuracy or utility of his inquiries. The first part of his paper gave us reason to hope, that the direction of the solar motion was about to be established; but in the conclusion of the paper, the point towards which the motion tends, is only approximately and by conjecture assigned, and stars retain their proper motions. Yet this indetermination has not arisen from any philosophic horror of hypotheses, from any scrupulous observance of the rules of just induction: the velocity of the sun is assumed; and, what must cause surprize, different distances are assigned to fixed stars. Arcturus is at the distance 1, Sirius at the distance 1,6809. Is not this assumption completely arbitrary? Indeed, after mature consideration, the positions and assertions of the author seem to rest on no foundation.

Art. 19. Observations on the singular Figure of the Planet' Saturn. By William Herschell, LL.D. F. R. S. p. 272.The indefatigable author of this paper has for many years contributed largely to the volumes of the Royal Society. Of late years, he has manifested a proneness to conjecture, theory, and hypothesis. In our opinion, that portion of his fame, which is destined to float down the stream of time will not be derived from his conjectural researches. We ejice, therefore, to find him, in the memoir before us, restored to his ancient and peculiar province, recording ob servations and making occasional inferences from them. The result of the observations now recorded, must surprize. the scientific world: we say the scientific world, because, to the generality of the world, the ring, the satellites, and the belts of Saturn, are much more an object of surprize and curiosity, than any deviation of the form of the planet from as phere or ellipsoid. A deviation from a spheroidical form,

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1

appears to result from Dr.H.'s observations, and is the subject of the present philosophic communication.

The equatorial diaineter of Saturn, according to the present and all preceding observations, is greater than the po lar: this is conformable to theory; but the equatorial diameter is not the greatest diameter: the greatest diameter is that which is drawn from latitude 45°. At this latitude, the curvature is greater than either at the poles or at the equator. Such at least is Dr. Herschell's account. Let us attend to his own words:

The figure of the planet is certainly not spheroidical, like that of Mars and Jupiter the curvature is less on the equator, and on the poles, than at the latitude of about 45 degrees. The equatorial diameter is however considerably greater than the polar.

In order to have the testimony of all my instruments on the subject of the structure of the planet Saturn, I had prepared the 40-feet reflector for observing it in the meridian. I used a magni fying power of 360, and saw its form exactly as I had seen it in the 10 and 20-feet instruments. The planet is flattened at the poles, but the spheroid that would arise from this flattening is modified by some other cause, which I suppose to be the attraction of the ring. It resembles a parallelogram, one side whereof is the equatorial, the other the polar diameter, with the four corners rounded off, so as to leave both the equatorial and polar regions flatter than they would be in a regular spheroidical figure.

The planet Jupiter being by this time got up to a considerable altitude, I viewed it alternately with Saturn, in the 10-feet reflector, with a power of 500. The outlines of the figure of Saturn are as described in the observation of the 40-feet telescope; but those of Jupiter are such as to give a greater curvature both to the polar and equatorial regions than takes place at the poles or equator of Saturn, which are comparatively much flatter."

The small table in which Dr. H. has registered liis obser vations is as follows, in proportional parts:

The diameter of the greatest curvature

The equatorial diameter

The polar diameter

Latitude of the longest diameter

36

358

32

43° 20′

The learned author observes, that the contents of his pa-per will lead to some intricate researches by which the quantity of matter in the ring, and its solidity, may in some degree be ascertained.

The researches must be intricate no doubt, in a question so extremely complicated. But at present, we indulge no expectation of soon seeing this phenomenon explained by the laws of physical astronomy. Without entering into a detailed examination, from arguments that obviously suggest themselves, this phenomenon of Saturn's figure seems ano

malous. If the matter of the ring attracts the matter in Saturn, ought not the parts in the same plane with the ring to be attracted from Saturn's center? We put the question with diffidence. There can be no mistake surely in Dr. Herschell's experiments, for they were frequently repeated and with different telescopes. Indeed, his character for accuracy of observation ought to guarantee him from such a suspicion. On such a subject, indeed, when calculations, laborious and intricate in the highest degree, will be founded on these new observations, inaccuracy would be unpardonable. So many curious appearances have been accounted for by the powers of analysis, in physical astronomy, that we dare not talk of despairing of the solution of this curious phenomenon. At present however, surprise is with us the predominant feeling, not unmixed with some trifling and obtrusive inquietudes concerning the justness and regularity of the laws of material attraction.

ART. XIII.-The Life of Thomas Dermody, interspersed with Pieces of Original Poetry, many exhibiting unexams pled Prematurity of genuine Poetical Talent; and containing a Series of Correspondence with several eminent Characters. By James Grant Raymond. 2 Vols. 8vo. 16s. Millar. 1806.

BIOGRAPHICAL memoirs of literary men have of late years been presented to (we had almost said obtruded on) the public in no inconsiderable profusion, in a great variety of style and manner, from the dignity of moral and philosophic instruction, to the insipidity of colloquial garrulity; and in as great a variety of exterior attraction, from the splendid quarto to humble twelves,.,

The general tenor of a studious life may be expected to exclude all striking and uncommon occurrences: these works are therefore found to partake in no small degree of a prevailing monotonous character, but little calculated to arrest the attention by variety of incident, or to interest the passions by detailing enterprizes of great pith and moment.'

Men who have distinguished themselves by their mental acquirements, and who have attained to great celebrity in the paths of science and literature, may be supposed to afford useful examples, both with respect to their habits of application, and their mode and course of studies.-When we are instructed by a display of profound and accurate knowledge, and delighted by an appropriate and graceful style, our cu riosity is naturally excited to inquire, by what depe of exertion these excellencies were attained, what portie: of them the possessor might owe to the bounty Care,