and the telescope proves it to be only an enormous collection of stars too distant to admit of their light being separately visible. The inference is, that " we are situated in an immense stratum of stars, the length and breadth of which is very many times greater than its thickness; so that when we look through the thin part we see separate stars in sufficient number, but when we look along the stratum we see smaller and smaller stars, more thickly placed than before, and terminating in those which are so distant that their united effect is that of a mere cloud."*— (See def. Galaxy, p. 54.)

"It is only when we look towards the MILKY WAY, that these bodies seem to retire indefinitely, and finally to be lost in a diffuse starriness. In all other places, the intervals between the luminaries are nearly quite dark, as if we were closer on the edges of our bed of stars, and therefore saw through it into the external and obscure vacancies space. The opinion is thus forced on us anew, that we are in the midst of a mere group or cluster of stars; and moreover, that it is a group of peculiar configuration, narrow, but greatly elongated in the line of the milky way.Nichol's Architecture of the Heavens, p. 13.

of

* Professor De Morgan.

DOUBLE STARS-BINARY SYSTEMS.

Of the myriads of stars, it was supposed until lately, that the brightest were the nearest; but the very small star marked 61 in the constellation Cygnus, has recently been selected for special observation, as being probably nearer than most of the rest. If this be so, it is evident that, of two stars of very unequal brilliance or magnitude, shining nearly together, and thus "optically double" in the apparent concave of the sky, the smaller may be millions of miles, or leagues, nearer to us than the brighter or larger.

Many stars too, which appear as single points of light to the naked eye, are double in the telescope; the individuals of such couples being, in some instances, equal in appearance, and in others, of very unequal magnitude.

H It was whilst unsuccessfully endeavouring to detect a variation of mutual distance between such a couple (i. e., a parallactic displacement of the nearer one of two such stars, in consequence of the earth's orbitual motion), that Sir William Herschel was led to the interesting discovery that, in numerous instances, these double stars compose "binary systems ;" and that although so enormously distant from us as to be utterly unaffected with an annual parallax, they are gravitating towards each other, and moving in regular orbits around a common centre of gravity.

K.. Sun around sun, each perhaps accompanied with its train of planets and their satellites, closely shrouded from our view by the splendour of their respective suns, (4, p. 187,) and crowded into a space bearing hardly a greater proportion to the enormous interval which separates them, than the distances of the satellites of our planets from their primaries bear to their distances from the sun itself."Herschel's Astronomy, p. 394.

Unless the planets which may be thus invisibly circling around one of such a couple of suns, "be thus closely nestled under the protecting wing of their immediate superior, the sweep of their other sun in its perihelion passage

round their own, might carry them off, or whirl them into orbits utterly incompatible with the conditions necessary for the existence of their inhabitants."--Ibid. (See S, on

page 257.)

Sir William Herschel recorded the places of 500 double stars. It was the frequency of such apparent combinations as he proceeded in his search, that led his mind to the improbability of their being, as before supposed, mere appearances of juxta-position. Since his death, the labours of Sir John Herschel, Sir James South, and Professor Struve, have augmented this number to 6000! of which not less than 30 or 40 are ascertained to be revolving systems; and several have the exact forms of their ellipses of revolution determined, and even the periods in which they complete them computed, from the observations taken of their varying relative positions and mutual distances.

Of the stars familiar to those who are using the CelesGlobe :

Rigel (8 of Orion); Polaris (a of Ursa Major); Vega (u of Lyra) and Cor Caroli, are double, and (with the exception of Vega) not difficult to discern. There is also a beautiful triple star in the right fore-foot of Monoceros; and the minute star e of Lyra (next N E of Vega), which appears as two when examined with a pocket telescope, presents, in one of considerable power, each one of this couple as itself a double star, equally distant from a fifth probably associated in system with them!

But, of other familiar objects, the periods are ascertained :

In a of Gemini (Castor), the period of revolution is 253 years. In γ of Virgo*

In y of Leo

Ine of Boötes (Mirach)

629

* The individual stars composing this, are nearly equal. About the year 1718, they were distant six or seven seconds (an interval nearly equal to the apparent diameter of Mercury, or half that of Saturn); they are now not more than one second separated, and appear in a moderate telescope as one elongated star: they will of course become separated on the other side after many years. The orbit of π of Serpentarius, is presented quite edgewise towards us; the one star of this system is now projected on the other, so that the largest and best telescopes cannot discern more than one star; but they were viewed separately by Sir William Herschel.

The following, from their shortness of period, have already, since their discovery, accomplished the greater portion of a course :—

In of Ursa Major (Mizar), the period of revolution is 58 years. In of Cancer

In 70 of Ophiucus or Serpentarius

55

801/

Whilst the next is even far advanced in its second

period ;

Inn of Corona Borealis

43 years.

VARIABLE STARS.

M Other evidences, not less palpable, of activity and system in the immensely distant regions of space, are noted in the appearances of new stars, as well as in the total disappearance of some, and the variation of brightness, or apparent magnitude, of others.

The star 42 of Virgo, as many before, was first missed on the 9th of May, 1828, by Sir John Herschel.

In 1572 a new bright star suddenly appeared in Cassiopeia, which, without perceptibly deviating from its place, increased in brilliance until it surpassed that of Sirius or Jupiter, being perceptible at noon; and finally disappeared by mere gradual diminution of splendour.

It is conjectured that this star is periodical: that it had been seen about 300 years before, and may, therefore, again appear in 40 or 50 years from the present time.

As late as 1670, a new star was discerned in the head of Cygnus, which after becoming invisible made its appearance a second time, and continued two years, till it died away.

Algol (B of Perseus) varies, with regularity, from the second magnitude to the fourth in three hours and a half; and in three hours and a half more has regained the splendour of the second magnitude to keep it undiminished for two days and fourteen hours.

It is supposed that this is caused either by the periodical obstruction of some dark planetary body in motion around it; or by the rotation of the star itself, which may thus alternately present a less bright portion of its surface.

Sir John Herschel gives a list of the variations, and periods of variation, of 15 stars.

Separation of the Individual Stars of a Binary System.

Suppose an individual, who fires a gun at A whenever he passes it, be engaged to traverse, with uniform speed, a circular or elliptical path A, B, C, D, and to be required to discharge a B musket at the instant of his arrival at C, without slackening his pace. Suppose, now, the exact time occupied in performing this task from A to A again to be 40 minutes and 10 seconds; but that the sound of the distant musket at C be heard by E not after an interval of half that time, or 20 minutes and 5 seconds from his gunfire at A; but after 20 minutes and ten seconds, -at which time it is evident that the traveller has been 5 seconds on his way from C towards D.

A

E

The listener at E knows, from this circumstance, the distance C A, and consequently, the extent of the path and the rate of the traveller. For, it is evident that the sound of the musket occupied five seconds in its passage from C across to A, and sound is known to travel 1,142 feet per second.

NA method somewhat similar to this, but depending on the fact that light travels with a certain velocity, (R, p. 254,) has been proposed by Mons. Savary, to afford a knowledge of the distance which separates the two stars of a binary system. Let the one star at A be watched throughout its revolution around the other, F. When at A it will be blended with F, and both will appear as one star to an inhabitant of the earth at E (supposed to be at a great distance). It will appear most separated when at B-be again blended with F when it arrives at C-again attain its greatest apparent separation at D, &c.

We will suppose this revolution of apparent blendings and separations to be performed in 62 years. If in the year 1800 the star was at A, in 1831 it was at C; but suppose observation to have ascertained that, although its third blending at A will be in 1862, it appeared to be blended by its arrival at C, not in 1831, but in a corresponding month of 1832? it will be evident (as in the other case) that the light which proceeded from the greater distance C, to announce its arrival there, had been just one year in travelling across to A; and since light travels nearly 100 millions of miles in eight minutes, it might hence be inferred by what interval these companion luminaries are separated!