Venus, when in a favourable position, is to us the most conspicuous member of the planetary system. Its apparent diameter differs considerably at different times: this is owing to the greater extent of variation of distance than occurs in the case of Mercury. Its phases are similar to those of Mercury, to which the reader is referred. The discovery of the phases of Venus is due to Galileo, who announced the fact to his friend Kepler in the following logograph or anagram: –

These things not ripe, as yet concealed from others, are read by me.

Which, when transposed, becomes —

Little is known of the appearance of this planet's surface, owing to its dazzling brilliancy when viewed in a telescope; notwithstanding this, however, spots are occasionally visible. These spots are not supposed to be connected with the planet's surface, but rather to belong to its atmosphere. Schröter has suspected the existence of numerous high mountains on its surface. The rotation of Venus on its axis was discovered by D. Cassini in the year 1667.

TheDominical Letter, called also the Sunday Letter, is an expedient by means of which we can readily find out the day of the week on which any day of the year falls, knowing the day of the week on which New Year's Day falls. To the first seven days of January are affixed the first seven letters of the alphabet—A, B, C, D, E, F, G; and the one of these which denotes Sunday is the Dominical letter. Thus, if Sunday is New Year's Day, then A is the Dominical letter; if Monday, that letter is G; and so on. If there were 364 days, or 52 weeks exactly in the year, then the Dominical letter would always be the same; but as the year contains aboixt 365¼ days, or 1¼ more than 364., this excess has to be taken into account every year, and the ¼ makes a day in every 4 years; so that the Dominical letter falls backward one letter every common year, and two letters every Bissextile or Leap year. Knowing the Dominical letter, we can ascertain all the Sundays, all the Mondays, &c, in the year. The reason why Leap years have two letters may be thus explained: —Take, for example, the year 1860.

Shooting stars, although noticed in former times, have only within the last half century attracted any particular attention. This branch of the science may therefore be considered to be, comparatively, in its infancy. We must possess a long and carefully made series of observations before we are likely to be acquainted, with any degree of precision, with the physical nature of these objects. They were formerly considered to be merely atmospheric meteors, caused by the combustion of inflammable gases. This opinion has, however, now lost much, if not all, of its force, and they are now recognised as bodies which, although they become inflamed on coming in contact with the Earth's atmosphere, yet have their origin far beyond it.

It is now an established fact, that there is no night throughout the year on which shooting stars may be not seen; and that, on an average, from 5 to 7 may be noticed on a clear night every hour. These occasional meteors may be termed sporadic, in contradistinction to those swarms which appear at certain times of the year, and which are periodic.

On examining the heavens on a clear evening, when the Moon is not shining, we find here and there groups of stars which seem to be compressed together in such a manner as to present a dull cloud-like appearance: these are termed clusters and nebulæ, and are usually classed as follows: —

1. Irregular groups, visible more or less to the naked eye.

2. Clusters resolvable into separate stars with the aid of a telescope.

3. Nebulæ, for the most part irresolvable.

The latter are subdivided into: —

1. i. Annular Nebulæ.

2. ii. Elliptic Nebulæ.

3. iii. Spiral Nebulæ.

4. iv. Planetary Nebulæ.

5. v. Nebulous Stars.

Of the 1st class, there are several examples to be found, with all of which the reader is probably more or less familiar. The cluster of the Pleiades in Taurus is probably best known. When examined directly, few persons can see more than 6 stars, but by turning the eye sideways, we discover there are many more, and between 50 and 60 are visible in a telescope. The most brilliant star in the group is Alcyone, or η Tauri, of the 3rd magnitude ; next in order come Electra and Atlas of the 4th; Maia and Taygeta of the 5th; Pleione and Celeno, which are between the 6th and 7th; Asterope between the 7th and 8th; and finally, a great number of smaller stars.

Time is, strictly speaking, of infinite duration; we are, therefore, obliged to choose some arbitrary unit by means of which a measurement of time may be effected. For short intervals, the diurnal rotation on its axis of the globe we inhabit; for longer intervals, the annual revolution of the Earth around the Sun, are the standards of measurement we employ; but any event which takes place at equal intervals of time may serve the purpose of a chronometrical register. Thus, the ages of certain trees may be ascertained by counting the number of concentric rings in the trunk, so many being formed annually; the ages of certain cattle, by the number of rings on the horns; the ages of horses may in like manner be ascertained by the successive disappearance of marks from their teeth; so also the pulsations of the heart, the flowing of a certain quantity of water from one vessel to another, the oscillations of a pendulum, may all be used as measurers of time: but, in practice, the length of a day is a natural interval of time, which the domestic habits of man force upon him; and accordingly we find that amongst all nations this unit of measurement is, under some form or other, the one adopted.

Between the orbits of Mars and Jupiter there is a wide interval, which, until the present century, was not known to be occupied by any planet. The researches of late years, as previously intimated in Chapter II., have led to the discovery of a numerous group of small bodies revolving round the Sun, which are known as the Minor Planets, and which have received names taken chiefly from the mythology of ancient Greece and Rome.

The planets differ in some respects from the other members of the system, especially in point of size, the largest being probably not more than 200 or 300 miles m diameter. Their orbits are also much more inclined, as a general rule, than the orbits of the older planets, whence they are sometimes termed the ultra-zodiacal planets.

The following is a list of these planets, together with the chief elements of their orbits:—

It is needless to give any detailed account of each, but a short summary may not be out of place.

The nearest to the Sun is Flora, which revolves round that luminary in 1193 days or 3¼ years, at a mean distance of 209,819,000 miles.

The most distant is Maximiliana, whose period is 2343 days or 6.4 years, and whose mean distance is 329,000,000 miles.

Previous to the celebrated eclipse of 1858, the Astronomer Eoyal circulated a useful series of suggestions, which we here reproduce, in a slightly amended form, as applicable to other eclipses besides the one above referred to: —

“ The following suggestions are offered as presenting grounds for consideration, which may tend to direct observers ia deciding on the employment of the means which they may possess.

“ I. Observations not requiring Instruments.

1. “ 1. As the eclipse advances, it is desirable to obtain some notion or measure of the degree of darkness.

2. “ 2. At what distance from the eye can a book or paper, exhibiting type of different sizes, be read?

3. “ 3. Hold up a lighted candle nearly between the Sun and your eye. At how many sun-breadths' distance from the Sun can the flame be seen?

4. “ 4. If you are in an elevated position, remark the changes of colour and appearance of the surrounding objects in the landscape.

5. “ 5. If you see the spots of light formed by the intersecting shadows of the boughs of trees, remark whether they exhibit the lunefonn of the Sun.

6. “ 6. When the annulus is formed, you will probably observe it with a darkened glass; but you are particularly requested to devote one instant (as early as possible) to the verification of this point, viz.: When the annular Sun is viewed with the naked eye, does it appear an annulus or a fully illuminated disc?

Foremost amongst the great clusters with which we are acquainted stands the Milky Way, which has pre-eminently occupied the attention of philosophers from the earliest ages of antiquity.

The course of the Milky Way amongst the constellations is thus sketched by Sir J. Herschel, whose description we shall quote with a few verbal alterations.

Neglecting occasional deviations, and following the line of its greatest brightness, as well as its varying breadth and intensity will permit, its course conforms nearly to that of a great circle inclined at an angle of about 63° to the equinoctial, and cutting that circle in E.A. 0h. 47m., and 12h. 47m.; so that its northern and southern poles respectively are situated in E.A. 12h. 47m., Decl. N. 27° and E.A. 0h. 47m., Decl. S. 27°. Throughout the region where it is so remarkably subdivided, this great circle holds an intermediate situation between the 2 great streams; with a nearer approximation, however, to the brighter and continuous stream, than to the fainter and interrupted one. If we trace its course in order of right ascension, we find it traversing the constellation Cassiopeia, its brighter part passing about 2° north of the star δ of that constellation, i. e. in about 62° of north declination. Passing thence between γ and ε Cassiopeia?, it sends off a branch to the south preceding side, towards α Persei, very conspicuous as far as that star, prolonged faintly towards e of the same constellation, and possibly traceable towards the Hyades and Pleiades as remote outliers.

If, on some clear evening, the reader will take the trouble to station himself on the summit of any rising ground, and cast his eye upwards, he will see the sky spangled with countless multitudes of brilliant specks of light; these are the fixed stars (we shall presently see that this appellation is not strictly correct); an attentive observer will soon notice, also, that the stars he is contemplating seem to revolve in a body around one of their number situated in the north, about midway between the horizon and the zenith; this is the Pole-star, so called from its being near the pole of the celestial equator. On account, however, of the precession of the equinoxes, the present Pole-star (α Ursæ Minoris) will not always be so; the true pole is now about from this star; this distance will be gradually diminished until it is reduced to about half a degree; it will then increase again, and after the lapse of a long period of time, the pole will depart from this star, which will then cease to bear the name or serve the purposes of a Pole-star. 3,970 years ago, the star γ in the constellation Draco fulfilled this office; 12,000 years hence, it will fall to the lot of a brilliant star of the 1st magnitude — Vega (α Lyræ) — which is 24° 52' from the pole.

Refraction. — Besides the change of place, to which the heavenly bodies are subjected in consequence of the effects of parallax, atmospheric refraction gives rise to a considerable displacement; and it is this power which the air, in common with all transparent media, possesses, which renders a knowledge of the constitution of the atmosphere very important to the astronomer. “ In order to understand the nature of refraction, we must consider that an object always appears in the direction in which the last ray of light comes to the eye. If the light which comes from a star were bent into 50 directions before it reached the eye, the star would nevertheless appear in a line described by the ray nearest the eye. The operation of this principle is seen when an oar, or any stick, is thrust into the water. As the rays of light by which the oar is seen have their direction changed as they pass out of water into air, the apparent direction in which the body is seen is changed in the same degree, giving it a bent appearance— the part below the water having apparently a different direction from the part above.”

Saturn is undoubtedly the most interesting member of the planetary system, not only from its being accompanied by 8 satellites, but especially on account of the system of rings by which it is surrounded. Belts are also occasionally noticed, but they are far more indistinct than those of Jupiter, though doubtless due to the same physical cause, whatever that may be. Spots are rare. Sir W. Herschel considered this planet to be surrounded by a very dense atmosphere; an idea that has been fully confirmed by subsequent observers.

When this planet was first telescopically examined by Galileo, he noticed that it presented a very oval outline, which he conjectured was owing to a larger planet having on each side of it two smaller ones. He added, that with telescopes of superior power, the planet did not appear triple, but exhibited an oblong form, somewhat like the shape of an olive.

Continuing his observations, the illustrious astronomer was not long in noticing that the two (supposed) bodies gradually decreased in size, though still in the same position as regards their primary, until they finally disappeared altogether.

The form of the Earth is not strictly spherical, the polar diameter being less than the equatorial by about 26½ miles; it is, in fact, like many, probably all, the planets, an oblate spheroid. The great circle of the heavens apparently described by the Sun every year, owing to our revolution round that body, is called the ecliptic, and is usually employed by astronomers as a fixed plane of reference. The Earth's equator prolonged in the direction of the fixed stars, differs from the equator of the heavens, which is inclined to the plane of the ecliptic at an angle which in January 1, 1860, was equal to 23° 27′ 33″ and which angle is known as the obliquity of the ecliptic. It is this inclination which gives rise to the vicissitudes of the seasons during our annual revolution round the Sun. The two points where the celestial equator is intersected by the ecliptic, are called the equinoxes; the points exactly midway between these being the solstices. It is from the vernal (or spring) equinox, that right ascensions are measured along the equator and longitudes along the ecliptic.

The class of bodies which will now come under our notice are among the most interesting with which the astronomer has to deal. Appearing suddenly in the nocturnal sky, and often dragging after them tails of immense size and brilliancy, they were well calculated, in the earlier ages of the world, to attract the attention of all, and still more to excite the fear of many. It is the unanimous testimony of history, during a period of upwards of zooo years, that comets were always considered to be peculiarly “ ominous of the wrath of heaven, and as harbingers of wars and famines, of the dethronement of monarchs, and the dissolution of empires.” We shall hereafter examine this question at greater length. Suffice it for us, here, to quote the words of the Poet, who speaks of

However little attention might have been paid by the ancients to the more ordinary phenomena of nature (which, however, were pretty well looked after), yet certain it is, that comets and total eclipses of the Sun were not easily forgotten or lightly passed over; hence the aspect of remarkable comets that have appeared at various times have been handed down to us, often with circumstantial minuteness.

Every inhabitant of a maritime country like Great Britain is more or less familiar with the phenomena now under our consideration, but beyond knowing the general fact, that the Moon has something to do with the tides, their physical history is not so well understood as it ought to be.

The phenomena of the tides are very frequently attributed to the attraction of the Moon, whereby the waters of the ocean are drawn towards that side of the Earth on which our satellite happens to be situated; in fact, that it is high water when the Moon is on or near the meridian of the place of observation.

This, though to a certain extent true, by no means adequately represents the facts of the case, for high water is not only produced on that side of the Earth immediately under the Moon, but also on the opposite side at the same time. The two tides are therefore separated from each other by 180°, or by a space equal to half the circumference of the globe. The diurnal rotation of the Earth, causing every portion of its surface to pass successively under the tidal waves, in about 24h., it follows that there are everywhere 2 tides daily, with an interval of about 12 hours between each; whereas, if the common supposition were correct, there would be only.

Of the total eclipses which have of late years been systematically observed, that of July 18, 1860, is by far the most interesting and important: it owes its interest to the agreeable circumstances connected with it, hereafter to be more fully spoken of, and its importance, to the very extensive and refined observations which were made by so many astronomers in America, Europe, and Africa.

Our limits wholly forbid our entering into any very lengthened statement: we shall therefore select from the published accounts of the observations, such portions as we deem most fitted to be placed on record in a work like the present, prefacing them by a brief epitome of the general circumstances attending “ the Himalaya Expedition.”

On Nov. 15, 1859, the Astronomer Royal, in an interview with the Duke of Somerset, First Lord of the Admiralty, drew his Grace's attention to the then approaching eclipse, at the same time suggesting the desirability of a ship being appropriated for the conveyance of observers to and from the coast of Spain. After the request was duly considered, Her Majesty's Government volunteered to place at the disposal of the Astronomer Royal and his friends, H.M.S. “ Himalaya”; the offer was of course gratefully accepted, and in due course of time the expedition set forth.

This planet was discovered by Sir W. Herschel on March 13, 1781, whilst he was engaged in scrutinising some small stars in Gemini. He observed one of them which seemed to have a more sensible diameter than the others, and to be less luminous. The application of high magnifying powers rendered these peculiarities more perceptible: he therefore made some very careful observations, and found that it was moving at the rate of 2¼″ per hour. He then announced to the Royal Society the discovery of a new comet, so little was a planet expected. Maskelyne found it, and soon suspected its true nature. On proper inquiries being made, it was found that Flamsteed had seen it 3 times, Mayer, once, and Le Monnier, 11 times previously to the epoch of Herschel's discovery; all of whom had been ignorant of its real nature.

A brisk discussion took place on the name the new planet was to have. Herschel himself desired to call it the Georgium Sidus in compliment to his friend and patron, our most excellent Sovereign King George III.; some of the foreign astronomers, amongst whom was Laplace, insisted that it ought to bear the name of its discoverer; Bode proposed Uranus as the mythological father of Saturn; and this name finally triumphed, though for a long course of years it was frequently known as the Georgian Planet.