Oldalképek
PDF
ePub

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 inere hypothesis resting on no other ground than that of slight probability, and the hypothesis is still weaker with respect to the notion of those stars that change their magnitudes periodically; for we must first suppose that these changes arise from ibeir rota. tory motion, and secondly that their rotatory motion is a sign or symptom of their motion of translation. . But recourse is bad to considerations of greater weight than theoretical considerations. If a slar appears to have a motion, such inotion inay 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 wbich 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 molions will tend lo a point in opposition to the direction of thal motion ; whereas all real inotions will be in.

discriminately scattered in space, Sr. With these distinctions in view, (says, Dr. H.) we may examine · the propet 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 motious; 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 dieete 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 be 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 lale ter 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 tirst magnitude, in a very limited part of the leavens, about the constellation of Hercules. Upon all the remaining surface of the same globe, there is not the least appearance of any other than a promiscuous sitnation of intersections; and of these only a single

one is made by arches of principal stars.' 1 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 inotions 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 die observed proper nio. tions of sars; that is, by resolving such proper inotions ei. ther into inere parallactic ones, or into motions compounded of parallactic aud 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 froin 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 continoed, in eet in a point, to be called their parallactic center; the opposile 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 lo a person situated at a star 90 degrees distant from the apex of the solar motion, and at a distance from the sun equal i, the sun shall appear annually to describe an arch = ?.",8-1995 ; fronsuch an hypothesis, and by the aid of a forinula, lhe parallactic motions of Arcturus and · Sirius are calculated, and these are found to agree with the proper motions established by observation,

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, apd its intersection with the curye of the proper motion of one of the preceding stars, such intersection ought to give the same solar apex as has been already delerinined. But if a third ştar, be taken, and the intersection determined, the sapex of solar inotion is not the sanie, and consequently the - motion of this third star is not parallactic solely, but is com· pounded of a real and of a parallactic motion. But if this

third star (Capella) have a proper molion, why should not Sirius and Arcturus? And if they have, is not the forher 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 sew that, when we are in search of an apex for the solar motion, it cught to be so fixed upon as to be

@jually favourable to every star which is proper for dirrcting our i chuice. Hence a problem will arise, in our present case, how to

and a poivt whose situation anong 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 front any one of the stars, its parellscuc' moriut, which is as the sine of

that distance, will be affecied: so that it is not the mere alterations, 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 tliem, in order to proceed to a more goneral 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, staled the arguments and investigation of the ingenious astronomer, rather from respect to hiş fame and talents than from conviction either of the accu. racy 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 pa. per, the point towards which the motion tends, is only approximately and by conjecture assigned, and stars relain, their proper motions. Yet this indetermination has not arisen froin 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 pot this . assumption completely arbitrary? Indeed, after mature con. : sideration, 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. 279.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 Hoat down the stream of time will not be derived from his conjectural researches. We jej sice, therefore, to find him, in the memoir before us, re siored to his ancient and peculiar province, recording oberie servations and making occasional inferences from them... The result of the observations now recorded, must surprize 3 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 froin as phere or ellipsoid. A deviation from a spheroidical form,

Transactions for 1805. Part II. 311 di'

B! appears to result from Dr.H.'s observations, and is the subject of the present philosophic communication. ; .: *15*!

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 dia. meter is not the greatest diameter: the greatest diameter is. that which is drawn from latitude 45o. At this latitude, the curyature is greater than either at the poles' or at the equa." tor. Such at least is Dr. Herschell's account. Let us at: tend to his own words:

• The figure of the planet is certainly not spheruidical, 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 hai prepared the 40-feet reflector for observing it in the meridian. I used a magnies fying power of 360, and saw its form exactly as I had seen it in v the 10 and 20-feet instruments. The planet is flattened at the poles, but the spheroid that would arise from this Hatlening is modified by some other cause, which I suppose to be the attraction of the ringe It resembles a parallelogram, one side whereof is the equatorial, the other the polar diameter, with the four corners rounded off, so as 10, leave both the equatorial and polar regions fatter 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 reflecta or, 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 equalor of Saturn, which are comparatively much flatter.':

The small table in which Dr. H. has registered liis obserai vations is as follows, in proportional parts : n'ersi

The diameter of the greatest curvature : 1.369. i The equatorial diameter

' .'". 35 ... The polar diameter ...

. 32

1 Latitude of the longest-diameter : 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 présent, we indulge no expectation of suon 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

maloys. If the matter of the ring attracts, the matter in Satúrn, ought not the parts in the same place 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, labopious and intricate in the higbest degree, will be foundedot these new observations, inaccuracy would be unpardonable. Şo.many curious appearances hare been accounted for by the powers of analysis, in physica! astronomy, that we dare not talk of despairing of the solution of this curious plieno. menon. At present however, surprise is with us the predominant feeling, not unpixed with some trifling and obtrue. sive inquietudes concerning the justness and regularity of the laws of material attraction.

[ocr errors]

ART. XIII.- The Life of Thomus Derniorly, interspersed

with Pieces of Original Poetry, many exhibiting uncran-i pled Prematurity of genuine Poetical Talent; and containing a Series of Correspondence with several eminent Chardcters. * By James Grant Raymond. 2 Vols. 8vo. '1hs. Millár. 1906.

BIOGRAPHICAL memoirs of literary meu have of late: years been presented to (we had almost said obliuded on) the public in no inconsiderable profusion, in a great variety · of style and manner, from the dignity of moral and philoso

phie instruction, to the insipidity of colloquial garrulity; and in is great a variety of exterior attraction, from the splendid quarto to bumble 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 monotojious character, but litile calculated to arrest the attention by varieiy. of incident; or to interest the passions by detailing 'enterprizes of great pith and moments

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 babits of application, and their mode and course of studies.--\Vhen we are i structed by a display of profound and accurate knowledge, and delighted by an appropriate and graceful style, ou che riosity is naturally excited to inquire, by what happens of exertion these excellencies were altained, which is of théin the possessor might owe to the bounds Live free

« ElőzőTovább »