been supposed to give evidence of a thin resisting medium, or ether, in the inter-planetary spaces.

1066. The constitution of the comets is still very obscure. The latest suggestion is that they are vast showers of small bodies of the kind named meteoric stones-a suggestion founded on some remarkable coincidences of comets with meteoric showers, or shooting stars. On this supposition Professor Tait shows what would be the consequence of the encounter of a comet with the earth, which is often spoken of as a possible casualty. We often pass through the tails of comets, if the present doctrine be correct, and the effect is a display of shooting stars like the November meteors. But if we were to pass through the nucleus or head, which is the densest part of a comet in a parabolic orbit, we might be so furiously bombarded with aërolite boulders that "there would be a wholesale massacre of living beings, and destruction of buildings and cultivated land over half the globe." But the probability of the occurrence is excessively small.

The source of light of the comets is still very mysterious. It is not wholly dependent on the solar light, and yet it is not wholly independent, for it increases as the comet approaches the sun.

Meteors and Aerolites.

1067. Among the most familiar appearances at night are the shooting stars, falling stars, or meteors. Often they disappear and leave no trace, but in some instances they are followed by the fall of solid material to the earth. This may be a mere dust shower, or it may consist of solid masses of great variety of sizes, called aërolites. A fine collection of them may be seen in the British Museum. Many are massive blocks weighing several hundredweights. The largest on record is one in Brazil, estimated at fourteen thousand pounds weight.

In composition the aërolites are for the most part largely made up of iron, with small quantities of other metals, especially nickel and cobalt, with copper, tin, manganese, chrome, and molybdenum. They also contain oxygen and carbon, and by means of Sprengel's vacuum Graham succeeded in extracting from them a quantity of hydrogen gas. The material constituents are therefore only those that we find in the earth, but in the form and manner of combination they differ from any terrestrial substance. They are usually covered with a thin black crust, and are very hot when they fall.

The periodicity of the meteoric swarms has been established in

The Constellations.

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several instances. The most remarkable epochs of their appearance are from 12th to the 14th of November, and on the 10th of August.

The average height of shooting stars has been estimated at sixty miles. This would be about the beginning of our atmosphere; and the friction of the air might be the cause of their blazing up.

That small masses or blocks of solid matter fly through space in countless millions is a sure inference from the facts. That these masses are gathered into vast swarms, groups, or streams is the probable interpretation of their periodic appearance; not, however, to the exclusion of smaller groups or isolated individuals. The connection of these swarms with comets has been noticed in the previous section.

SECTION VI.

The sun himself is the nearest to this earth of innumerable self-luminous bodies, existing through illimitable space, which, owing to their distance, appear to our sight very small. These are the STARS.

1068. The bodies apart from our solar system are commonly spoken of as the stars or the fixed stars, because they appear to us to keep their places with relation to one another, and do not shift or wander like the planets, satellites, and comets.

The fixed stars are the bright points of the nocturnal sky. They are of different degrees of brightness, and from that circumstance are classified into stars of the first, second, and other magnitudes. The smallest of those visible to the naked eye are of the sixth magnitude. The number of the visible stars is about two thousand for one half of the heavens, or four thousand for the entire sphere. The stars not seen by the naked eye and discovered by the telescope are vastly numerous; they are classified down to the seventeenth magnitude.

From very early times the stars have been grouped into figures er forms called constellations. They are named after men, animals, and fanciful objects. Twelve such surround the heavens in the neighbourhood of the ecliptic, and are called the twelve signs of the Zodiac: Aries (the ram), Taurus (the bull), Gemini (the twins), &c. North of the Zodiac are enumerated twenty-one constellations. Perhaps the most notable is Ursa Major, the Great Bear, in which

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Distances of the Stars.

are the seven conspicuous stars named the Plough. South of the Zodiac are fifteen constellations, many of which never appear to us : one, named Orion, is apparent at a certain season of the year.

Individual stars are designated, according to their relative brightness, by Greek letters and by numbers, under their several constellations, as a, ẞ, y, Ursa Majoris; 61 Cygni (the Swan).

1069. To know the real sizes of the starry bodies, the first thing is to find their distances. The ordinary methods of finding distance by parallax were for a long time unavailing. For although the diameter of the earth's orbit, 183 millions of miles, was taken as the base line, no sensible parallax could be observed. Now, a parallax of 1", or the displacement of a star by that quantity, when viewed from opposite positions of the earth in her orbit (that is, at a six months' interval), corresponds to a distance of 19 billions of miles, or more than two hundred thousand times the distance of the sun. Such a parallax is within the means of observation, although it is doubtful whether smaller amounts can be relied on. The first star believed to have an annual parallax was 61 Cygni, estimated by Bessel at about a third of a second. Next was one of the brightest of the southern stars, a Centauri, whose parallax was estimated by Henderson at very nearly a second; the latest estimate being o'91, or about nine-tenths of a second. The corresponding distance would be 21 billions of miles. For the quickest movement in nature, the movement of light, to traverse this distance would take three and a half years.

If this be one of the distances of the others.

nearest stars, we may judge what are the It is not certain that greater brightness means greater nearness; one star may differ from another in glory," or in size and light-giving power.

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The star a Centauri is three times as bright as our sun would be at the same distance.

1070. A singular fact concerning the stars is, that in regard to brilliancy, some of them are variable, hence called the variable stars. They go through periods of increase and diminution. Two or three hundred of such have been noticed. The cause may be either the intervention of a dark body, or the unequal manifestation of the light and dark parts of the surface, corresponding to our solar spots. Also there are instances of stars coming into view for a time, and then vanishing.

While in the multitude of the stars there must be every degree of proximate or apparent position, cases have been discovered of

couples changing their relative position as if by a proper motion, on the part of one or both. These are the Double Stars, first discovered by the elder Herschel. Periods of revolution have been assigned in some instances. The double star 70 Ophiuchi accomplishes a revolution in about 80 years. Sir W. Herschel observed 2400 double stars. There are groups of more than two, called Multiple Stars. There are now known upwards of 6000 double and multiple stars. In them we have an extension of our ideas of the grandeur of the universe. Besides systems made up of a central sun with its attendant pianets, we are presented with two or more suns revolving in the same system, by which their planets might enjoy an almost perpetual day.

1071. The farther discovery has been made that the stars are not absolutely fixed, but have in many instances a proper motion in space. No doubt this motion is very slow, otherwise it would lead to displacements that would have been discoverable long since. The question is naturally suggested: Is our sun fixed in space? As the effect of his moving would be to open up the stars that he approached towards, and make closer those that he receded from, there is a means of determining the fact. It is actually shown that he is steadily advancing towards a certain point in the heavens located in the northern constellation Hercules.

The spectroscope employed upon the stars has shown that they do not essentially differ in constitution from the sun. As in the sun, hydrogen is a prevailing element. Other substances identified are sodium, magnesium, calcium, iron, bismuth, antimony, mercury, tellurium.

While many of the stars are of a brilliant white colour, there are great varieties of colour among the rest. There are numerous shades of red, yellow, and blue, which would seem to show great differences in their constituent materials. The spectroscopic observations also show the same variation in the predominating elements.

1072. Nebula.-In various parts of the heavens are discerned cloudy or hazy patches, which have given rise to much study Many of these patches, when examined by powerful telescopes, have appeared to be clusters of stars, while others have preserved their nebulous aspect. It was not unreasonable to suppose that these last also, under still more powerful telescopes, might appear to be nothing but dense masses of stars. But there are now grounds for thinking that there are irresolvable nebulæ, or collections of diffused

hazy light, indicating a peculiar celestial phenomenon, explained as uncondensed stars. Observed by the spectroscope, these nebulæ are seen to contain glowing hydrogen gas.

The star clusters are suggestive of many reflections as to the constitution of the material universe. They would seem to indicate the existence of detached star-systems, or galaxies, self-contained, and possessing innumerable suns (each perhaps with planets) performing movements among themselves under gravitation. It is conjectured that our sun is a member of the huge galaxy, appearing as the milky way (a flattened form, with a cleft in the edge), which contains the great mass of the visible stars. The nebulæ (star clusters) occur in all parts of the heavens, and represent other galaxies like our milky way, strewn in the immeasurable depths of space.