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CHAP. XIII. round its axis, would also displace its center, and give it a motion of translation in absolute space

Conse-
of

"If the sun has a motion in absolute space, directed toward any such a the quarter of the heavens, it is obvious that the stars in that quarter must

quences

ory.

stars.

Double

appear to recede from each other, while those in the opposite region would seem gradually to approach, in the same manner as when walking through a forest, the trees toward which we advance are constantly separating, while the distance of those which we leave behind is gradually contracting. The proper motion of the stars, therefore, in opposite regions, as ascertained by a comparison of ancient with modern observations, ought to correspond with this hypothesis; and Sir W. Herschel found, that the greater part of them are nearly in the direction which would result from a motion of the sun toward the constellation Hercules, or rather to a part of the heavens whose right ascension is 250° 52′ 30′′, and whose north polar distance is 40° 22′. Klugel found the right ascension of this point to be 260°, and Prevost made it 230°, with 650 of north polar distance. Sir W. Herschel supposes that the motion of the sun, and the solar system, is not slower than that of the earth in its orbit, and that it is performed round some distant center... The attractive force capable of producing such an effect, he does not suppose to be lodged in one large body, but in the center of gravity of a cluster of stars, or the common center of gravity of several clusters." The following figures, taken from Norton's Astronomy, represent the telescopic appearance of some of the double stars.

"There are stars which, when viewed by the naked eye, and even and multiple by the help of a telescope of moderate power, have the appearance of only a single star; but, being seen through a good telescope, they are found to be double, and in some cases a very marked difference is per-ceptible, both as to their brilliancy and the color of their light. These Sir W. Herschel supposed to be so near each other, as to obey reciprocally the power of each other's attraction, revolving about their common center of gravity, in certain determinate periods.

Revolutions

Castor, Leonis, Rigel, Pole Star, Monoc, Cancri. "The two stars, for example, which form the double star Castor, of the multi- have varied in their angular situation more than 450 since they were ple stars. observed by Dr. Bradley, in 1759, and appear to perform a retrograde revolution in 342 years, in a plane perpendicular to the direction of the sun. Sir W. Herschel found them in intermediate angular positions, at intermediate times, but never could perceive any change in their distance. The retrograde revolution of y in Leo, another double star, γ is supposed to be in a plane considerably inclined to the line in which we view it, and to be completed in 1200 years. The stars of Bootes,

perform a direct revolution in 1681 years, in a plane oblique to the sun. CHAP. XIII. The stars of Serpens, perform a retrograde revolution in about 375 years; and those of in Virgo in 708 years, without any change of their distance. In 1802, the large star of Hercules, eclipsed the smaller one, though they were separate in 1782. Other stars are supposed to be united in triple, quadruple, and still more complicated systems.

"With respect to the determination of the real magnitude of the stars, Description and their respective distances, we have as yet made but little progress. of nebula. Researches of this kind must be left to future astronomers. It appears, however, that the stars are not uniformly distributed through the heavens, but collected into groups, each containing many millions of stars. We can form some idea of them from those small whitish spots called Nebulæ, which appear in the heavens as represented in the accompanying illustration. By means of the telescope, we distinguish in these collections an almost infinite number of small stars, so near each other, that their

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rays are ordinarily blended by irradiation, and thus present to the eye only a faint uniform sheet of light. That large, white, lumi

nous track,

which traverses the heavens

from one pole to

the other, under

the name of the Milky Way, is probably nothing but a nebula of this The Milky kind, which appears larger than the others, because it is nearer to us. Way a neWith the aid of the telescope we discover in this zone of light such a bula. prodigious number of stars that the imagination is bewildered in attempting to represent them. Yet from the angular distances of these stars, it is certain that the space which separates those which seem nearest to each other, is at least a hundred thousand times as great as the radius of the earth's orbit. This will give us some idea of the immense extent of the group. To what distance then must we withdraw, in order that this whole collection may appear as small as the other nebula which we perceive, some of which cannot, by the assistance of the best telescopes, be made to present anything but a bright speck, or a simple mass of light, of the nature of which we are able to form some idea only by analogy? When we attempt, in imagination, to fathom this abyss, it is in vain to think of prescribing any limits to

CHAP.

XIII. the universe, and the mind reverts involuntarily to the insignificant portion of it which we are destined to occupy.

Observa

ble II.

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Before we close this chapter, we think it important to call the attentions on ta tion of the reader to table II, in which will be seen, at a glance (in the columns marked annual variation), the general effect of the precession of the equinoxes; and although we have called particular attention to the fact elsewhere, we here notice that all the stars, from the 6th to the 18th hour of right ascension, have a progressive motion to the southward (—), and all the stars from the 18th to the 6th hour of right ascension have a progressive motion to the northward (+), and the greatest variations are at 0 h. and 12 h. But these motions are not, in reality, the motions of the stars; they result from motions of the earth. Whenever the annual motion of any star does not correspond with this common displacement of the equinox, we say the star has a proper motion; and by such discrepancy it has been decided, that those stars marked with an asterisk, in the catalogue, have proper motions; and the star 61 Cygni, near the close of the table, has the greatest proper motion.

The paral. From this circumstance, and from the fact of its being a double star, lax of 61 it was selected by Bessel as a fit subject for the investigation of stellar Cygni discoparallax ; and it is now contended, and in a measure granted, that the annual parallax of this star is 0".35, which makes its distance more than 592.000 times the radius of the earth's orbit; a distance that light could not traverse in less than nine and one-fourth years.

vered.

SECTION III.

PHYSICAL ASTRONOMY.

CHAPTER I.

GENERAL LAWS OF MOTION THE THEORY OF GRAVITY.

CHAP. I.

(148.) In a work like this, designed for elementary in- What should struction, it cannot be expected that a full investigation of be expected physical astronomy shall be entered into; for that subject in this work. alone would require volumes; and to fully appreciate and comprehend it, requires the matured philosopher combined with the accomplished mathematician.

We shall give, however, a sufficient amount to impart a good general idea of the subject—if one or two points are taken on trust.

For elementary principles we must turn a moment to natu- Elementary ral philosophy, and consider the laws of inertia, motion, and principles. force. Motion is a change of place in relation to other bodies which we conceive to be at rest; and the extent of change in the time taken for unity is called velocity, and the essential cause of motion we denominate force.

measure of

A double force will give a double velocity to bodies moving Velocity the freely in void space, or in an unresisting medium-a triple force. force, a triple velocity, &c. This is taken as an axiom - and hence, when we consider mere material points in motion, the relative velocities measure the relative amounts of force.

There are three elements to motion, which the philosopher never loses sight of; or we may say that he never thinks of motion without the three distinct elements of time, velocity, and distance, coming into his mind.

Algebraically, we put t, v, and d, to represent the three elements, and then we have this important and general equation,

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CHAP. I. Expression for force.

The law of inertia.

Some central

act on the

the

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(149.) As forces are in proportion to velocities (when momentum is not in question), therefore, if we put ƒ and F to represent two forces corresponding to the distances d and D, which are described in the times t and T, then by making use of equation (2), in place of the velocities, we have

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(150.) A body at rest, has no power to put itself in motion; and having no self power, no internal force or will, in any shape, it cannot increase or diminish the motion it may have, or change the direction it may be moving. This is the law of inertia. It cannot of itself change its state; and if it is changed it must be acted upon by some external force; and this accords with universal experience; and this law is the most natural and simple of any we can imagine, but it is only in the motion of the heavenly bodies that it is fully exemplified.

The earth, moon, and planets move in curves not in force must right lines. The directions of their motions are changed. motions of Something external from them must, therefore, change them; earth, for the law of inertia would continue a motion once obtained moon, and planets. in a straight line. Now this force must exist within the orbit of every curve; we therefore naturally refer it to the body round which others circulate. The earth and planets go round the sun, and if we could suppose a force residing in the sun to extend throughout the system sufficient to draw bodies to it, this would at once account not only for the planets deviating from a right line, but would account for a constant deviation of all bodies to that point, and the preservation of the system.

The moon's

The moon goes round the earth, constantly deviating from motion con- the tangent of its orbit, and the law of inertia is constantly

sidered.

* We number the proportions the same as equations, for a proportion is but an equation in another form.

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