water, as compared with pure water at 62° Fahr., is 1.0275; and this density, corresponding as it does with an average saltness of 3 per cent, will decrease where the water is fresher and increase where it becomes salter than the general average. This saltness and density renders the ocean less vaporisable than fresh water, and also keeps it longer from freezing-the freezing point of fresh water being 32° Fahr., while that of sea-water is 280. This composition is all-essential to the wants of its plants and animals; and though the rivers are incessantly carrying in fresh accessions of saline matter, the equilibrium is ever maintained by these wants as well as by the chemical interchanges that take place among its sediments.

The average depth of the ocean is estimated at 4 miles, and reliable soundings have been taken at nearly 5 miles; but, as a large portion is much under the general average, it is highly probable that some of its recesses may sink to the depth of 8 or 10 miles. So far as we know, the ocean-bed has its deeps and shoals, its pits and precipices, its troughs and ridges, very much like the surface of the dry land. In fact, it is but the submerged surface of former lands; and no better conception can be formed of its irregularities than by standing on some lofty mountain, and supposing the hills and valleys, the glens and gorges, the plains and plateaux that lie beneath, to be covered with water. A knowledge of these depths—or rather of the shoals and irregularities of the ocean-bed-is of vast importance in geography, at the same time that it is all-essential to navigation. The safety of the commerce of the civilised world depends upon correct chartography; these deeps and shallows determine the direction of currents, and inset and velocity of tides; and marine life is visibly regulated both in its dispersion and numerical amount by depth and mineral conditions of sea-bottom.

The surface temperature of the ocean varies of course with the latitude, being highest at the equator, and gradually decreasing towards either pole. In the torrid zone temperatures varying from 78° to 88° have been noted, and from these maxima it declines, stage by stage, to the perpetual ice of the polar regions. The mean temperature has been ascertained to be 39° Fahr., or nearly that of water at its maximum density; and between the 56th and 57th parallels (S. lat.) this temperature has been found to be uniform from the surface to the greatest depths. Towards the poles, however, the surface becomes colder, and the mean of 39° in the 70th parallel is not found till at the depth of 750 fathoms; while towards the equator the surface grows warmer, and the mean of 39° is not reached till at the depth of 1200 fathoms.

Colour, luminosity, and the like, are generally local and seasonal peculiarities depending upon the presence of mineral or organic impurities which may occur in one area and not in another, or during one brief season, and yet be absent for the greater portion of the year.

To those who desire more extensive details of the ocean and its physical peculiarities, we cannot recommend them to a more instructive or attractive source of information than Captain Maury's Physical Geography of the Sea'-a volume replete with reasonings as well as observation, though occasionally impeded by reflections which impair, rather than impart to, the Ivalue of its information.

IX.

THE WATER--ITS OCEANS AND OCEAN-CURRENTS.

Waves--their Height, Velocity, and Impact.

124. LIKE other fluids whose particles are free to yield to every impulse, the waters of the ocean are subject to several movements, the more important of which are waves, tides, and currents. Waves are produced by winds, and occasionally by earthquake commotion; tides by the attractions of the moon and sun; and currents chiefly by that incessant tendency which waters of different densities and temperatures have to assume a state of equilibrium. And, first, of WAVES, whose main characteristics are magnitude, velocity, and force of impact. Waves occur in every part of the ocean, and wherever the wind blows,-the aërial current drifting the surface waters along with it, and producing undulations, which increase, according to the power of the propelling force, from the gentlest ripple to billows 30 or 40 feet in height. In deep and open seas a continuous wind produces merely an undulation or up-and-down movement of the surface waters; and this commotion, even in the case of a wave a quarter of a mile in breadth and 40 feet high, is not sensibly felt at a depth of 220 fathoms. But in obstructed and shallow seas the lower part of the advancing undulation is retarded by frictional contact with the bottom-the upper portion advances with headlong motion, and ultimately breaks with forcible impact on the opposing shore, which is worn and abraded by the backward and forward motion of the surf. Such is the beginning, course, and termination of ordinary waves, at first a mere ripple; as the gale increases, a long roll and swell in the deep sea; and ultimately a cresting and dash of breakers on the shelving shore. Occasionally, however, the wind shifts, and sets in waves from opposite directions, and these crossing and commingling produce

I

a violent commotion, even far out at sea, and in the deepest

waters.

125. The aspects and characters of waves are known to sailors by many different names; the ruffle or ripple under a rising breeze being spoken of as a catspaw, the long undulation as a swell or billow, the shorter undulations as they approach the shore as rollers and breakers, and the broken water along shore as surf. The big heavy waves that occasionally set in when there is no wind (having been produced by storms far out at sea), are said to form a ground-swell; the commotion produced by cross-waves forms a chopping sea, and in a less degree a jabble or cross-lipper; but these and similar terms belong more to nautical technicality than to the generalities of Physical Geography. In circumscribed and shallow seas waves are short and abrupt; in the open ocean they assume the character of a long rolling swell. Whatever diminishes the friction of the wind-packs of ice, floating sea-weed, oil, and the like-suppresses the rise of the waves; and even during fogs and rains the sea is not so rough as in dry weather, in consequence of the diminished attraction of the atmosphere for water, and necessarily lessened concomitant friction.

126. Generally speaking, the magnitude of wind-waves has been greatly exaggerated, partly from the difficulty of making correct observations, and partly from the impression of dread produced on the mind of the observer. The greatest waves known are said to be those off the Cape of Good Hope, where, under the influence of a north-west gale, they have been found to exceed 40 feet in height. Off Cape Horn they have been measured at 32 feet from trough to crest; and in the North Atlantic, waves from 20 to 25 feet are by no means uncommon. In our own seas, however, they rarely exceed 8 or 10 feet; and all accounts of their running "mountains high" must be received as mere poetical exaggerations. In the case of earthquake-waves the conditions are altogether different; and as the whole mass of water is then thrown into commotion by sudden and abrupt risings, fallings, and whirlings, waves, or rather walls of water, 60 or 80 feet high, may be thrown with tremendous impetus upon the land. In the Lisbon earthquake of 1755, the destructive wave that rolled in upon the coasts of Portugal was estimated at 60 feet; and in the Simoda (Japan) earthquake of 1854, three huge waves, at intervals of a few minutes, rushed into the bay, destroying the native craft, and completely submerging the town of Simoda.

127. The velocity of waves depends primarily, of course, upon the power and continuance of the wind, but is greatly modified by, and bears an ascertainable relation to, their magnitude and

the depth of the water over which they travel. Thus it has been calculated by Professor Airy that a wave 100 feet in breadth and in water 100 feet deep travels at the rate of about 15 miles per hour; one 1000 feet broad and in water 1000 feet deep, at the rate of 48 miles; whereas another, 10,000 feet in breadth and in water 10,000 feet deep, will sweep forward with a velocity of not less than 154 miles per hour. This relation between the breadth of a wave, its velocity of progress, and the depth of the water in which it travels, has been embodied by Mr Airy in the following table:

100,000

2.262

7.151

1793.300

22.264 71.543 226.260 715.430 1688.300

128. The force with which a wave strikes against any opposing barrier depends, in like manner, upon its bulk and velocity; and in the case of huge waves this impact is enormous. From experiments made at lighthouses and breakwaters, their effective pressure has been estimated as high as 6000 lb. per square foot and one has only to observe the breaches occasionally made in seawalls, and the distance to which blocks of stone, several tons in weight, have been hurled forward, to be convinced of their great propulsive power. Of course the force with which a wave simply strikes is not to be altogether estimated by its propulsive power, for substances submerged in water lose a certain portion of their weight, which greatly facilitates their displacement and transport.

Tides-their Origin and Influence.

129. The next, and perhaps the most important and persistent of oceanic movements, is that of the TIDES-a term applied to the periodic rising and falling of the waters, occasioned chiefly by the attraction of the moon, but partly also by that of the sun. In obedience to the universal law that "every particle in nature attracts every other particle with a force inversely as the square of the distance," the earth is attracted by the sun and moon, but more by the latter, in proportion to its greater proximity. Land