seconds, so that each flow and each ebb occupies nearly six hours and a quarter. When the flow reaches its maximum height it is said to be high water; when the ebb arrives at its minimum elevation it is said to be low water.

Causes. The tides are caused by the attraction of the waters of the ocean by the sun and moon. Though the sun is much larger than the moon, the latter has a greater attractive force on the waters of the earth, because its distance is vastly less. Hence the influence of the moon in the formation of the tides is much more powerful than that of the sun. The period of 24 hours 50 minutes and 28 seconds is the length of time which any given spot on the earth's surface, carried by its diurnal revolution, takes to come back to the same relative position with regard to the moon as it occupied at the commencement of the period.

The attraction of the sun and moon is of course exerted on the whole mass of the earth; but since the water has a smaller cohesive power than the land, it yields more readily to the force applied to it, and is heaped up on that side of the globe which is turned towards the attracting bodies.

In the accompanying diagram let E represent the earth, S the sun, and M the moon. The waters of the globe are drawn towards P by the joint attraction of the sun and moon acting in

the same direction along the line E S. In this position, where the sun and moon are both on the same side of the earth, their attractions exert their maximum influence on the waters of the earth, because they act in the same direction.

The student will readily see that the water is now raised at P, but he will not so easily comprehend the fact we now mention; namely, that at the same time there is also high water at Q, on the opposite side of the world. The explanation is this; the water at P is raised by its being drawn away from the earth, whilst the water at Q is raised by the earth being drawn away from it. The earth is attracted most at P, less at the centre E, and less still at Q. Hence the water at Q is, as it were, left behind by the other parts of the globe, and is heaped up on that side at the same moment that it is elevated on the other side, nearest to the sun and moon. Thus each of the two bodies, the sun and the moon, tends to produce a flow of the tide both on the side nearest to itself and also on the opposite side at the same moment.

The highest tides will therefore occur when the sun and moon are on the same side of the earth, as at S and M, or on opposite sides of the earth, as at S" and M. The former position occurs at the time of new moon, when the sun and moon are said to be in conjunction; the latter position occurs at the time of full moon, when the sun and moon are said to be in opposition. These highest tides are called Spring Tides; they happen twice during each lunar period. The spring tides do not however occur exactly at the times of new and full moon, but about a day and a half afterwards, since the waters require some time to obey fully the forces applied to them.

It will be evident that when the flow of the tide rises highest at high water the ebb will fall lowest at low water, since an accumulation of water in one portion of the globe must be counterbalanced by a corresponding depression in that from which the water is withdrawn.

When the moon is at M and the sun at S', their attractive forces act along the lines M E and S' E, which are at right angles to each other. In this position the two forces in some degree

counteract each other; but the attraction of the moon being much greater than that of the sun, the waters obey the impulse given to them by the former, but to a diminished degree. The position referred to occurs when the moon is in her first and third quarters (at the time of half-moon), or in quadrature, as it is termed. The tides which happen at these times are the lowest; they are called Neap Tides.

The Tidal Wave. The movement of the waters of the ocean to obey the attractive forces of the sun and moon is called the tidal wave. It varies in force, direction, and velocity in different parts of the world on account of the obstacles offered to its progress by the masses of land it meets with in its passage. In the open ocean it moves rapidly and in a regular course, but in narrow and shallow seas its flow is slow and interrupted. On the other hand, for obvious reasons, the tide rises highest where its progress is most impeded, and the water is consequently more confined. It rushes with great force through narrow straits and up the gradually narrowing and funnel-shaped mouths of rivers, and it is in situations of this kind that it attains its greatest height. In the River Severn, for instance, the tide sometimes rises to a height of forty or fifty feet. In the Bay of Fundy, in North America, a height of more than 100 feet has been registered, while in the open waters of the Pacific the difference between high and low water is often not more than two or three feet. Sometimes the tide rushes up in a great wave which can be seen for miles before it arrives. This is called the Bore. At the mouth of the Brahmapootra, for example, the bore attains the height of about twelve feet. This phenomenon is also observed in the Hooghly, a branch of the Ganges, in the Bay of Fundy, in the mouth of the Severn, and other places.

WINDS. Winds are currents of air. They are created and regulated chiefly by the same causes as the currents of the ocean the influence of temperature and the rotation of the earth on its axis. Winds are of three kinds :

(1) Constant, those which blow regularly in the same direction, as the trade winds.

(2) Periodical, those which blow in different directions at different times of the year, according to fixed laws, as the

monsoons.

(3) Variable, those which blow in an irregular and uncertain manner, as the sirocco.

The velocity of the wind varies to a very great extent. When it moves at the rate of 6 or 7 miles an hour, it is called a gentle breeze; at 14 or 15 miles an hour, a stiff breeze; at 40 miles an hour, a high wind, or gale; at from 50 to 60 miles an hour, a storm; at from 80 to 100 miles an hour, a tempest, or hurricane.

The force of the wind depends of course on its velocity. In a great storm the strength of the wind is enormous. A terrible example of the immense force of the wind was afforded by the destruction of the Tay Bridge, at Christmas, 1879, when more than seventy unfortunate railway passengers perished in the storm which blew that structure into the Firth. It is calculated that a velocity of 10 miles an hour causes a pressure of half a pound on each square foot of surface; a velocity of 20 miles, two pounds; of 30 miles, three and a half pounds; of 40 miles, nearly eight pounds; of 50 miles, more than twelve pounds; of 80 miles, nearly thirty-two pounds; and of 100 miles, nearly fifty pounds.

The air near the equator being more heated than near the poles, it expands, becomes lighter than the colder air, and ascends. Hence, the equilibrium being destroyed, as in the case of the waters of the ocean, the cold air from the north and south rushes in and displaces the warmer air. Hence we have a north wind and a south wind from the poles both setting in towards the equator. But since the rotatory motion of the earth from west to east is slower near the poles than near the equator, and the air by which it is surrounded partakes of its motion, the slower air from the poles is left behind as it arrives nearer to the equator, and therefore the north and south winds become north-east and south-east winds respectively. Since the causes which produce these movements are continual in their opera

tion, the winds thus created are constant. They are called the Trade Winds, on account of the great advantages which navigators derive from the regularity of their course.

North of the equator the trade winds blow from the northeast; south of the equator they blow from the south-east. They extend to about 30° on each side of the equator. Between the trade winds, and nearly coincident with the equator, but rather to the north of it, is a narrow belt in which the two trade winds meet and neutralize each other. This part of the world is called the Region of Equatorial Calms, or Variables; by sailors it is termed the Doldrums. In this belt, which extends to a breadth of three or four hundred miles, rain, thunder-storms, long calms, and variable winds are frequent.

The trade winds blow with great regularity in the Atlantic Ocean. In the Pacific Ocean they are less regular, on account of the numerous islands, which create land and sea breezes, and thus interfere with the steady course of the trade winds. In the Indian Ocean the south-east trade wind is regular, but the north-east trade wind is interrupted by the great mass of land north of the equator.

Above the trade winds are the currents of warm air which have been replaced by the cooler air from the northern and southern regions, and which now make their way towards the poles. Becoming colder in the higher parts of the atmosphere, they at length become of a lower temperature than the air beneath them. Hence they descend to the surface of the earth at about 30° north and south of the equator, and since their motion from west to east acquired near the equator is greater than that of the regions to which they have arrived, their directions are towards the north-east and south-east; i.e., they are south-west and north-west winds. Hence in the portions of the earth north and south of the trade winds, or north of the Tropic of Cancer and south of the Tropic of Capricorn, the prevailing winds are from the south-west and north-west. These winds are sometimes called the Return Trade Winds.

Near the tropics, on both sides of the equator, where the