the case of these seas, and also of lakes, the Moon's influence is equally exerted; and, there being no swell, is less perceptible. Though we learn that the cause of the tides was first suspected by Pytheas, a navigator, who flourished 400 в. c., yet Newton was the first to demonstrate that the tides were the result of the attraction of the Sun and Moon; and thus did he add another to the long list of benefits which he conferred on posterity, by explaining those sublime and beautiful laws, according to which it has been ordained that the world shall be governed, by Him who

Gives and guides the Sun's attractive force,
And steers the planets in their silver course;
With heat and light revives the golden day,
And breathes his spirit on organic clay;
With hand unseen directs the general cause,
By firm, immutable, immortal laws.

Air, being lighter than water, and being likewise nearer to the Moon, must be more within her influence: hence there are doubtless aërial tides; of which if the nature were well understood, the subject of meteorology and of the weather generally, would be more decidedly a science than it now is.

LATITUDE AND LONGITUDE.

The ancients knew that a great boundary to the west was formed by the Atlantic Ocean; but the confines of the Earth towards the east they supposed were illimitable. Hence, the distance on the Earth's surface, measured from west to east, they termed Longitude, or measurement in length, which they supposed infinitely greater than the measurement in breadth, north and south, which they termed Latitude. Ptolemy, in the

middle of the second century of the Christian era, first used this mode of fixing the positions of places on the Earth's surface.

In the modern acceptation of the terms, latitude is the distance of any place north or south from the equator; and longitude, its distance east or west of any given meridian. Thus, the line of latitude is always at right angles to the line of longitude; and the spot, denoted by a certain latitude and longitude, is situated where these lines cross each other. This mode of marking out the position of a place on the surface of the Earth, is referrible to the motion of the Earth on its axis: which motion is the most constant and equable in nature.

In the subjoined figure, we have a representation of the terrestrial globe. The North and South Poles, P and Q, are the extremities of the imaginary axis; or

those points which do not revolve at all. The equator, EM BR, is equi-distant from these points. Lines parallel to the equator are called parallels of latitude; of which as many may be drawn as we choose; but it is usual in maps and globes to draw them through every 10° northward and southward: the space between every two parallels being then divided into ten parts, each of which is a degree. From P or Q to E or R are reckoned 90°, which is one quadrant of the globe.

It is usual for the principal nations in the world, which have cultivated science, and made themselves respectable in the opinion of fellow-nations, to reckon their longitude from the meridian of their own capital. Thus, the English reckon longitude from the meridian of Greenwich, which is, as it were, part of London, and is the site of the royal observatory. The French count degrees of longitude from the meridian of Paris.

Longitude is reckoned eastward and westward; and the meridians are usually drawn on globes and maps 15° apart, because the Sun passes over this extent of the Earth's surface in one hour; or all round the globe in twenty-four hours: so that there are twenty-four meridians, or lines of longitude, such as we see drawn from P to Q, where the line PAQ is the meridian of a. Each degree of longitude is, at the equator, about sixty-nine and a-half English miles; and each degree becomes less as we recede from the equator towards the poles, in consequence of the circles getting smaller.

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CHAPTER IX.

THE EXTERIOR PLANETS. MOTIONS OF THE PLANETSSTATIONARY, DIRECT, AND RETROGRADE. MARS. THE ASTEROIDS. JUPITER-MOONS OF THEIR USE IN

FINDING THE LONGITUDE AND IN DETERMINING THE

VELOCITY OF LIGHT.

URANUS-MOONS OF.

SATURN-RING AND MOONS OF.

In making observations upon the heavens, and the globes which exist and move about in them, the beholder, whether ordinary or telescopic, has his attention arrested at one time by the apparent size,—at another time by the brightness, then again by the colour, then by the sparkling, then by the steady light of the celestial object of his vision:-these circumstances, one or more, are found to apply to the planets spoken of in chapter 5, and likewise to the remaining planets, of which we are now about to speak. But most curious and singular above all, is the RING OF SATURN. To this nothing like or similar is found in the celestial creation; and so forcibly are we struck with this consideration, that, in passing beyond the orbit of the Earth, we have our mental vision directed to it at once, though in order, it is the last but one of the planets to be treated of.

It astonishes us that a star should be found at all thus encompassed, and still more does it astonish us that there should be but one star, as far as our ken has enabled us to judge, which has so elegant, but mysterious, an ornament. Its nature and uses are matters of speculation, and easily occur, as suggested by fancy, to

N

the mind of the reader of astronomy; but when the reader finds that there is a planet beyond Saturn, which not only has no ring, but even a smaller number of moons than Saturn, he will feel himself puzzled to account for this seeming discrepancy in the regulation of the planetary globes by the Author of good, unless he remember for a moment that full observations have not yet been made on the planet Herschel, or Uranus, owing to its immense distance, and the comparative imperfection of the instruments of man. To this we may append the consideration, that by means inscrutable to us the Almighty has, in all probability, compensated to other globes the deficiencies which their greater distance has occasioned them.

This consideration will apply likewise to the circumstance of no Moon being found to revolve round the planet Mars, which is further from the Sun than the Earth but here, the constitution of its atmosphere is probably such, that so much solar light is absorbed and refracted, that lunar or reflected light is not found wanting. But we cannot afford to linger any longer in the regions of speculation; let us pass on therefore to consider

THE EXTERIOR PLANETS.

Their wandering course, now high, now low, then hid,
Progressive, retrograde, or standing still,

In six thou seest.- -MILTON'S Par. Lost, book viii.

Before we enter into the details of the planets, whose orbits are without that of the Earth, we have to notice a peculiarity in their motions as seen from the Earth, which results from the circular or elliptical form of their