Ptolemy from observations of Hipparchus (B.C. 130); and this led to the suspicion-since fully confirmed that these at least, though called Fixed Stars, have a Real Motion. Observation proves that other stars are similarly movingamong them numerous Double Stars, the individuals of which are carried along together.

3. Q. At what Rates do the Stars thus Move?

A. On an average, the Proper Motion of the stars does not exceed 1" per year, though in some cases it is nearly 8". To move over a space equal to the Moon's Apparent Diameter, the Star 61 Cygni, whose Proper Motion is 5" 12, takes above 3 centuries; Alpha Centauri takes 5; and Sirius from 15 to 16. Yet the Actual Velocity represented by this Proper Motion amounts, in the case of Alpha Centauri, to about 980,000 miles a day; in that of Sirius, to nearly 2 millions; and in the case of 61 Cygni, to fully 2 millions.

4. Q. How far is the Sun affected by a like Motion?

A. The Sun is believed to have a Proper Motion and, with the planets, to be travelling towards a point in the constellation of Hercules, at the rate of 1.623 radii of the earth's orbit, or nearly 150 millions of miles, a year. This Solar Motion is shown by the observed tendency of the stars in certain directions to open out or separate, while they appear to close up in opposite directions; much as to a traveller in a forest the trees he approaches appear to separate, while those he leaves behind seem to crowd closer together.

5. Q. How has the Sun's Motion been accounted for? A. The Sun's Proper Motion is supposed to be

connected with the revolution of the whole Solar System, in a Period of millions of years, round some fixed Central Point or Body; and Mädler believes this Local Centre to lie in or near the group of the Pleiades. It is not, however, quite certain that the Solar System has a Proper Motion, and observations extended over centuries may be required to settle the question.

6. Q. To what Notable Changes are the Stars subject? A. Some stars, including Sirius, have changed their colour; many stars once visible are lost, and new ones have appeared; Temporary Stars, after blazing forth for a time, have died away; and Variable Stars-of which upwards of 100 are known, and to which Missing and Temporary Stars may belong are subject to periodical changes of brilliancy. The Variable Star Mira (Omicron Ceti), in the Whale's neck, takes 331 days to complete its changes; while Algol (Beta Persei), in Medusa's head, takes less than 69 hours. 7. Q. How do the Stars, Mira and Algol, vary?

A. Mira is at its greatest brightness-2nd magnitude for a fortnight, grows fainter for about 3 months, changes from white to red, and then disappears for 5 months. Algol, after appearing for 611 hours of the 2nd magnitude, fades to the 4th in about 3 hours, remains such for 15 minutes, and then regains, as rapidly as it lost, its brightness. Whether these changes are due to the star's rotation on an axis, to the periodic approach of a planet, or to peculiarities in the star's figure, or in the form of its orbit, is unknown (Les. 40). 8. Q. When have Notable Temporary Stars been seen? A. A Temporary Star, visible in the daytime,

appeared B.C. 125; Tycho Brahé described one which was seen from Nov. 1572 to March 1574 (probably the same that appeared in 945, 1264, and 1572): one seen by Kepler (1604-5) nearly equalled Venus in brightness; and another (T Coronæ) suddenly burst forth as if a-blaze in May 1866. The star described by Tycho Brahé suddenly appeared in Cassiopeia, as bright as Sirius; was successively white, yellow, red, and white again; became brighter than Jupiter; was visible at noon-day; then declined and disappeared (Lesson 40).

LESSON 37.-THE GALAXY, VIA LACTEA,
OR MILKY WAY.

1. Q. In what Region of the Heavens do the Stars appear most Numerous?

A. The stars of our firmament, which appear to be unequally scattered in different regions, increase rapidly in number towards the borders of that great zone or belt of diffused light called the Galaxy or Milky Way. Of the 20 millions of stars visible in powerful telescopes, no less than 18 millions lie in the neighbourhood of this zone of milky-looking light-the Galaxy-which stretches every night across the heavens, from horizon to horizon, and completely encircles the whole sphere (Lesson 33).

2. Q. What is the Situation of the Milky Way?

A. The Galaxy, which is inclined to the equinoctial at an angle of about 63°, traverses the constellations of Aquila, Cygnus, Cassiopeia, Perseus and Auriga; passes between the feet of Gemini and the horns of Taurus, and over the

club of Orion to Monoceros and Canis Major. Its breadth is, in some parts, only 3° or 4°, in others from 12° to 16°; its most northern point is in Cassiopeia, its most southern in Crux Australis ; and one of the most conspicuous portions of the Galaxy visible in Great Britain passes through Aquila and Cygnus (Lesson 33).

3. Q. Of what does the Milky Way consist?

A. Sir W. Herschel proved the Galaxy to consist of stars of all magnitudes-from those visible to the naked eye down to the minutest specks of light, only to be seen through the best telescopes― scattered by millions like glittering dust on the black ground of the general heavens. He supposed our stellar system to form a stratum, and the apparent crowding to arise from our looking up at the edge of the stratum from near the middle of its thickness; much as innumerable lamps arranged in the form of a flat ring would appear crowded together if seen from the centre in the direction of the ring's breadth.

4. Q. What is the supposed Form of this Star-stratum? A. Herschel supposed the sun and all the stars visible to the naked eye to belong to the Galaxy, to which he assigned a shape resembling that of a huge millstone, or flat ring, of immense length and breadth, but of comparatively small thickness. This ring, which is of irregular thickness and breadth, and is broken by dark gaps, sends out from its sides broad branches and narrow streaks of light. In one part, near which the sun is situated, the star-stratum opens out and presents a double edge, as if split into two great branches which reunite after remaining distinct for about 150°.

5. Q. Why are Fewer Stars seen away from the Galaxy?

A. The thickness of the starry stratum is small compared with its length and breadth, and in looking away from the Galaxy we look beyond the edge of the stratum. In a rich portion of the Milky Way Sir W. Herschel found 588 stars included at once in the field of view of his great telescope. By a process called "Gauging the Heavens," he counted the stars occurring in a single field of view-15' in diameter of his telescope, when directed to points in all parts of the visible heavens, and found the regions most distant from the Galaxy the poorest in stars (Lesson 38).

6. Q. How do Various Parts of the Galaxy Differ in Appearance, as seen through powerful telescopes?

A. Various portions differ much. One portion differs from another as to the number, magnitude, and distribution of the stars-rich clusters occurring in some parts and absolutely dark starless spaces in others. Sir W. Herschel saw 116,000 stars pass his field of view in 15 minutes, and in 41 minutes 258,000; while at the point of its nearest approach to the south pole, the Galaxy encloses a dark pear-shaped space-called from its blackness, the "Coal-Sack"-of about 8° long by 5° broad, in which only one star is visible to the naked eye (Lesson 38).

7. Q. What is Known of the Extent of the Galaxy? A. Nothing is certainly known of the Extent, or of the Exterior Form, of the Galaxy; but the moderate number of extremely minute stars met with in some regions, together with the perfectly black ground of the heavens between them, has led some to conclude that, there at least, the starry