retreating, it is at full tide; and when its recession has arrived at its lowest point, and it is about to advance again, it is at low tide.

Although the ebb and flow follow each other in regular succession, they periodically vary in degree; when higher than ordinary they are called spring, when lower, neap tides; the lowest ebbs invariably follow the highest floods, and vice versa.

There are so many difficulties attending the investigation of the causes by which this alternate action of the tide is produced, that although it has for a very long time engaged the attention of men of science, it has hitherto been accounted for only upon mere hypothesis. But whatever the absolute primal laws may be, by which this phenomenon is produced, there is no doubt that the influence of solar and lunar attraction upon the earth's surface are in some way connected with it.

The sun and moon, by the force of the attraction of gravity, have a tendency to draw the earth towards their centres; but owing to the great inferiority of the moon, both in size and gravitating power, to the earth, and the immense distance of the sun from it, their attractive force is only sufficient to draw towards them that portion of our globe which is directly under the zenith of their power. The greatest effect is produced by the influence of the moon, from its comparative nearness to the earth, and hence it is the chief agent by which the tides are produced.

The greater portion of the earth's surface is covered with water (only three-tenths being land). The particles of which fluids are composed, having less affinity to each other than those of solid bodies, are more easily influenced by attraction, and have consequently a greater tendency to yield to the moon's attraction, when any given spot is brought under her influence; but the gravity of the earth exercising a counter force to draw them back, only a high protuberance is raised, which forms a huge wave in the midst of the ocean. But as the force of gravity is directed in straight lines, the greatest influence will be exercised over those parts which come nearest under its centre: the greatest elevation of the wave, therefore, will be where it is nearest under the moon's meridian, and diminishing gradually towards the extremities,

north, south, east, and west, as the lines of attraction become more oblique.

A portion of the ocean being thus drawn up by the moon, the body of the earth immediately below it will also be similarly influenced, but (from its density, and from the power of gravity diminishing as the distance increases) in a much smaller degree. The waters on the other side the globe being subjected to a still less attractive power, the earth recedes from them, occasioning thereby an increase of their depth, so that a sort of second wave is formed at the exact antipodes of the other. The attractive influence of the sun is precisely of the same character as that of the moon, but very much less powerful, in consequence of its great distance.

There are thus four great tidal waves-two larger produced by the moon, and two smaller by the sun; the lunar waves being the agents by which the actual ebb and flow of the tides are effected.

According, however, to the relative position of the sun and moon, their respective waves either retard or accelerate each other; when the moon, for example, is in her syzygies, that is, in a direct line with the sun and earth, (as she is at the full, &c.) their combined influence is on the same spot, consequently the smaller waves are in conjunction with the larger; the size of the chief wave is increased, and the spring, or highest tides are produced. When, on the other hand, the moon is at right angles with the sun and earth, as in her quadratures (new moon, &c.), each wave is at its greatest distance, ninety degrees apart; and rising independently, they oppose each other, producing the lowest, or neap tides.

The earth being a globe, revolving upon its polar axis, it will follow that every place coming within the sun and moon's parallel, will, in rotation, be brought under their meridian, causing an elevation of the water, or high tide, at the spot over which they are passing, greater under the equinoxes, and gradually diminishing towards the north and south pole: hence the successive ebb and flow.

As the period of time the earth takes to make a revolution is once in twenty-four hours, fifty minutes, some seconds, (the two tidal waves invariably travelling at the antipodes of each other,) there will, of course, be two flood tides in that period at every place round the globe,

L

an interval of twelve hours twenty-five minutes (half the period of a revolution) elapsing between each. But as our calendar day consists of only twenty-four hours, it will follow that each tide will be twenty-five minutes later than the corresponding tide of the preceding day.

The shores of the land, however, intercepting the regular course of the primal wave, and causing it to separate and flow round, and into all the various interstices of seas, lakes, gulphs, and rivers, the tides will be later in arriving at the full, in places that are at a great distance from the main ocean. There are various other causes which operate upon the flow of tides in inland waters, as the direction of their course, their relative position, &c. In channels that have a north and south opening, the tide will sometimes flow twice in the course of a few hours, in consequence of one division of the great wave arriving at the corresponding opening of the channel before the other division has arrived at the other opening,-as, for instance, when it has been accelerated by winds or currents. In seas where the opening of communication is very small, there is scarcely any tide at all, because the advancing waters have not sufficient time to collect to make any perceptible increase before they again recede. Such is the case with the Baltic, the Mediterranean, and the Black Sea.

Thus are produced that ebb and flow of the tides, which, assisted by other agencies, keep up a continual motion of the waters of the earth, by which their stagnation, and consequently their decomposition, is prevented. -F. P. Nichols.

219

POETRY.

THE CONVOLVULUS MAJOR.

Most beautiful flower! 'tis delight to behold
Once more to the sunrise thy banner unroll'd,
As it greeted the gaze of the boy,

When first the soft tints in each delicate fold
I ponder'd with marvelling joy!

Still searching with eager inquisitive view

The bound that disparted thy crimson and white-
The thread that dissever'd them both from thy blue-
And still as more near the nice boundary drew,
The more it eluded the sight.

All worthy art thou to be flower of the child!
For he who would see thee in grace undefil'd,
Must rise with the childhood of day;

The dew-sprinkled hours of the morningtide mild
Alone can allure thee to stay:

Thou shrinkest and witherest as day travels on,

Like Childhood's sweet morn before oncoming years;

Like infant mortality, wasting alone,

The soul to her mansion in Paradise gone,
Ere the dew of the Font disappears.

Yet rural Philosophy, bending her ear,
The tones of inanimate Nature to hear,
Hath deem'd she discover'd in thine
Extinction of hopes, that no sooner appear
Than perish, nor wait their decline.*

In "The Language of Flowers," the Convolvulus Major repre

sents Extinguished Hope.

I trust not her tale-such illusions I know,

But never, bright flower, hast thou taught me to grieve, Whose briefness but likens thee more to the bow

That melts into heaven, 'mid the promise and glow
Of hope that can never deceive.

Bright page of the infinite volume, outspread
To all who will read it,-O, never unread
By me be one line of thy lore!

But still let me on by thy beauty be led,
To search thy calm wisdom the more:

That wisdom, whose clearness the simplest may see,
But sages must search, who would duly improve,
Where meet, like those exquisite colours in thee,
The soft-blended hues of the shade-seeking three,
Obedience, Humility, Love.

True wisdom, by learning and art unrefin'd,
That asks not the power or the pride of the mind;
Nay, flings to the scorner his scorn;

Plain wisdom, which all who seek early shall find,
As a child may find thee in the morn;

That boasts not its might-nay, avows itself weak,
And powerless to rise till with strength it entwine;
That hastes the support of the Rood-tree to seek:
Then, tenderly clasping it, patient and meek,
To burgeon with blossom divine.

Rev. Henry Thompson.

THE WAR OF THE LEAGUE.

Now glory to the Lord of Hosts, from whom all glories are!

And glory to our sovereign liege, King Henry of Navarre! Now let there be the merry sound of music and of dance, Through thy corn-fields green, and sunny vines, oh, pleasant land of France!

And thou, Rochelle, our own Rochelle, proud city of the waters,

Again let rapture light the eyes of all thy mourning daughters.