are nightly devoted to those measurements upon which the great truths of astronomy are mainly based. These instruments are made with every refinement of skill; but it is the duty of the painstaking astronomer to distrust the accuracy of his instrument in every conceivable way. The great tube may be as rigid a structure as mechanical engineers can produce; the divisions on the circle may have been engraved by the most perfect mechanical contrivance; but the conscientious astronomer will not rely upon mechanical precision. That meridian circle which, to the uninitiated, seems a marvellous piece of workmanship possessing almost illimitable accuracy, is presented in a different light to the astronomer who makes use of it. No one can appreciate, indeed, so fully as he, the skill of the artist who has made it, and the numerous beautiful contrivances for illumination and reading off, which give to the instrument its perfection; but while he recognises the beauty of the actual machine he is using, the astronomer has always before his mind's eye an ideal instrument of absolute perfection, to which the actual meridian circle only makes an approximation. Contrasted with this ideal instrument the best meridian circle is little more than a mass of imperfections. The ideal tube is perfectly rigid, the actual tube is flexible ; the ideal divisions of the circle are all perfectly uniform; the actual divisions are not uniform. The ideal instrument is a geometrical embodiment of perfect circles, perfect straight lines, and perfect right angles; the actual instrument can only give us approximate circles, approximate straight lines, and approximate right angles. Perhaps the spider's part of the work is on the whole the best; he gives us the nearest mechanical approach to a perfectly straight line; but we mar his work by not being able to put in his beautiful threads with perfect uniformity, while our attempts to stretch two of them across the field of view at right angles, do not succeed in producing an angle of exactly ninety degrees. Nor are the difficulties encountered by the meridian observer solely due to his instrument. He has to contend with his own want of skill; he has often to allow for personal peculiarities of an unexpected nature; the troubles that the atmosphere can give

him are notorious; while the levelling of his instrument tells him that he cannot even rely on the solid earth itself. The meridian circle shows that the earthquakes, which sometimes startle us, are merely the more conspicuous instances of incessant and universal movements in the earth, which every night in the year derange the delicacy of the instrument.

When the existence of these errors has been recognised, the first great step has been taken. By an alliance between the astronomer and the mathematician it is possible to measure the differences and the irregularities which separate the ideal meridian circle from the actual meridian circle. Once this has been done, it is possible to estimate the effect which all the irregularities can produce on the observations, and finally, to purge the observations from the grosser errors with which they are contaminated. We thus have observations, not indeed mathematically accurate, but still close approximations to those which would be obtained by a perfect observer, using an ideal instrument of geometrical accuracy, standing on an earth of absolute rigidity, and viewing the heavens without the intervention of the atmosphere.

It is not, however, necessary to use such great instruments as those just described in order to obtain some idea of the aid the telescope will afford in showing the celestial glories. The most suitable instrument for commencing astronomical studies is within ordinary reach. It is the ordinary binocular that a captain uses on board ship; or if that cannot be had, then the common opera-glass will answer nearly as well. This is, no doubt, not nearly so powerful as a large telescope, but it has some compensating advantages which the telescope does not possess. The opera-glass will survey a large region of the sky at once, while a telescope only looks at a small part of the sky. Let us suppose that the observer is provided with an opera-glass and is about to commence his astronomical studies.

The first step is to become acquainted with the very renowned group of seven stars which is represented in Fig. 8. It is often called the Plough, but astronomers prefer to regard it as a portion of the constellation of the Great Bear (Ursa Major).

There are many features of interest in this constellation, and the beginner should learn as soon as possible to identify the seven remarkable stars. Of these the two stars, a and B, at the head of the bear are generally called the "pointers." They are of special use in astronomy, because they enable us to find out the most im portant star in the whole sky, which is known as the "pole star." We shall return in a later chapter to the study of the different constellations. Our present object is a simpler one; it is merely to employ the Great Bear as a means of teaching us how vast is

the richness of the heavens in stars. Each student of astronomy is recommended to make one very simple observation on the Great Bear, which will give a wondrous conception of what the telescope can do, and will also reveal in a very impressive manner the glories of the starry heavens.

Fix the attention on that region in the Great Bear bounded by the four stars a By S. They form a sort of rectangle, of which the stars named are the corners. The next fine night try and count how many stars are visible within that rectangle. There are no really bright stars, but there are two or three sufficiently bright to be easily seen. On a very fine night, when there is no moon, perhaps a dozen might be perceived, or even more, according to the keenness of the eyesight. But when the opera-glass is directed to the same region, a most interesting, and indeed astonish

ing sight will be witnessed. Instead of the few stars which were seen before with difficulty, a hundred stars or more can now be seen with the greatest ease. The opera-glass will, indeed, easily disclose ten times as many stars as could be seen with the unaided eye.

But even the opera-glass will not show nearly all the stars in this region. Any good telescope will reveal hundreds of stars too faint for the opera-glass. The greater the telescope, the more numerous the stars; so that in one of the colossal instruments this region would be found studded with thousands of stars.

We have chosen the Great Bear for the purpose of this illustration, because it is more generally known than any other constellation. But the Great Bear is not exceptionally rich in stars; any other part of the sky would equally well have demonstrated the grand truth, that the stars which our unaided eyes disclose, are only an exceedingly small fraction of the entire number with which the whole heaven is teeming. To tell the number of the stars is a task which no man has accomplished; but various estimates have been made. Our great telescopes can probably show at least 50,000,000 stars. There would be a star apiece for every man, woman, and child in the United Kingdom, and there would still remain a liberal margin for distribution elsewhere.

The student of the heavens who uses a good refracting telescope, having an object-glass about three inches in diameter, will find ample and delightful occupation for many a fine evening. He should also be provided with an atlas of the stars, while a copy of the "Nautical Almanac," and of Webb's "Celestial Objects for Common Telescopes," will form a sufficiently complete astronomical equipment for much interesting occupation.

CHAPTER II.

THE SUN.

The vast Size of the Sun-Hotter than Fusing Platinum-Is the Sun the Source of Heat for the Earth ?-The Sun is 92,700,000 miles distant-How to realise the magnitude of this distance-Day and Night Luminous and Non-Luminous Bodies-Contrast between the Sun and the Stars-The Sun a Star-The Spots on the Sun-Changes in the Form of a Spot-They are Depressions on the SurfaceThe Rotation of the Sun on its Axis-The Size and Weight of the Sun-Is the Sun a Solid Body ?-View of a Typical Sun-Spot-Periodicity of the Sun-Spots -Connection between the Sun-Spots-Terrestrial Magnetism-The Faculæ— The Granulated Appearance of the Sun-The Prominences surrounding the Sun-Total Eclipse of the Sun-Size and Movement of the Prominences—Drawings of the Objects, coloured-The Corona Surrounding the Sun-The Heat of the Sun.

IN commencing our examination of the orbs which surround us, we naturally begin with our peerless sun. His splendid brilliance gives him the pre-eminence over all other celestial bodies. The proportions of the sun are commensurate with his importance.

Astronomers are actually able to measure the sun; and they find that his dimensions are so great as to tax our imagination to realise them. The diameter of the sun, or the length of the axis, passing through the centre from one side to the other, is 865,000 miles. Yet this bare statement of the dimensions of the great globe fails to awaken an adequate idea of its vastness. If a railway were laid round the sun, and if we were to start in an express train moving sixty miles an hour, we should have to travel night and day for five years without intermission before we had accomplished our journey.

If the sun be compared with the size of the earth, its stupendous bulk becomes still more apparent. Suppose his globe were cut up into one million parts: each of these parts would appreciably exceed the bulk of our earth. Were the sun placed in one pan of a