The interval or time which elapses from the time of the moon's transit over the meridian of a place to that of high water next following at the same meridian, is called the "luni-tidal interval."

It is found in general that any particular tide is not due to the moon's transit immediately preceding, but to a transit which has occurred some time before, and which is said, therefore, to correspond to it. The interval between the transit of the moon, at which a tide originates, and the appearance of a tide itself, is called the "retard" or "age" of the tide.

The directions of strong winds, as well as the varying pressure of the atmosphere, considerably affect both the times and the heights of high

water.

If the actions of the sun and moon were uninterrupted by obstacles or forces of other kinds, the tides would be regular and their calculation certain.

But from the unequal depth of the ocean and the barriers presented by continents which stand across the natural progress of the tides, their motion is interrupted, and the tide-wave, abandoned by the forces which originated it, becomes subject to the mechanical action proper to waves in general.

TIDE AND HALF-TIDE, TIDE AND QUARTER-TIDE.

The interval between high and low waters in the open sea, about six hours and twelve minutes, is designated a "tide."

In channels where a tidal stream is formed, when the stream continues to flow up for three hours after it is high water, it is said to make a "tide and a half-tide;" if it continue to flow for about one hour and a-half after high water, it is said to make a " tide and a quarter-tide."

The tides which take place far up bays, sounds, and rivers, are later than the tides at the entrance of such inlets, but they are not more irregular; on the contrary, the tides in such situations are often remarkably regular.

The tide in its progress up inlets and rivers is often much magnified or modified by local circumstances. Sometimes it is magnified so that the wave which brings the tide at one period of its rise advances with an abrupt front of broken water. This is called a "bore" (as in the Severn and Amazon). Sometimes the tide is divided into two half-day tides in its progress up the river, as in the Forth in Scotland. In all cases, after a certain point, the tide dies away in ascending a river.

The velocity of the tide-wave appears to be determined mainly by the

depth of water through which it is propagated. According to Airy's rule, "The rate at which the tide travels is equal to the velocity acquired by a body falling through space from a height equal to half the depth of water." The range depends mainly upon the variations in width, and upon the configuration of the coast.

The velocity of the tide-wave, by the above rule, is as follows:

Besides the acquaintance with the periods of high and low water, and the comparative heights of the day and night tides, it is necessary to observe the direction of the stream of flood and ebb, and the time at which it turns; but care must be had not to confound the time of the turn of the tidal stream with the time of high water. The turn of the tidal stream generally takes place at a different time from high water, except at the head of a bay or river. The stream of flood usually runs for some time, often for hours, after the time of high water. In the same way the stream of ebb runs for some time after low water.

The most violent tidal currents which occur in navigable channels on our coast are those of Hell Gate, New York, and those near the junction of Vineyard and Nantucket Sounds.

Very little is known of tidal currents on outside coasts, except in the immediate neighborhood of certain dangerous shoals. A knowledge of them would, however, be often a great advantage in the saving of time in the pas sages of coasters.

Coasters frequently lose their reckoning, in quiet and thick weather, by being swept out of their courses by these drifts. The coast currents, in some places, have a velocity of a third of a mile per hour in thirty fathoms of water. The epochs of coast currents may be expected to differ widely

from those of the local tide, especially on a shore where differences of tidal range appear from point to point.

As an aid to navigation, it is very desirable to observe and tabulate the currents" on soundings."

TIDES AND TIDAL CURRENTS OF THE UNITED STATES.

The Atlantic tides are of the most ordinary type, ebbing and flowing twice in twenty-four hours, and having but moderate differences in height between the two successive high waters or low waters, one occurring before noon, and the other after noon. The coast presents in its general outline three large bays: the southern from Cape Florida to Cape Hatteras, the middle from Cape Hatteras to Nantucket, and the eastern from Nantucket to Cape Sable, known as the Gulf of Maine. The tide-wave arrives at about the same time at the headlands, Cape Florida, Cape Hatteras, Nantucket, and Cape Sable, and at those points the height is inconsiderable, compared with the rise at the head of the various bays. At Cape Florida the range is only one and one-half feet, at Hatteras but two feet, while at the intermediate entrance to Savannah it reaches seven feet. Again, at the head of the middle bay, in New York Harbor, it reaches five feet, while at Nantucket Island it is little over one foot. The recess of Massachusetts Bay is marked, the increase in height reaching ten feet at Boston. Rolling on along the coast of Maine, it steadily increases, and a striking effect of the convergence of shores is exhibited in the Bay of Fundy, which opens its bosom to receive the full wave. At St. John, the mean height of the tide is nineteen feet, and at Sackville thirty-six feet, swelling to the enormous height of sixty feet, and even more, at the highest spring tides.

GULF OF MAINE.

Tidal currents at entrance. Along the line between Nantucket Shoals and Cape Sable Bank the ebb-current runs southwardly during the first four and a half hours after the northing or southing of the moon, and the floodcurrent northwardly from the sixth to the eleventh hour after the northing or southing of the moon. The time of turning on George's Bank corresponds nearly with the time of high or low water at Boston and Portland, but in the channel to the westward of the Bank, it is later, and to the eastward earlier.

TIDES IN NEW YORK HARBOR.

Between seven and eight hours after the moon's transit, high water has just passed Sandy Hook with an elevation of about four and a half feet, and at the same time has advanced just inside Block Island with a height of two feet. The latter tide, traversing the Sound with increasing velocity and height, reaches Sand's Point about eleven and a half hours after the transit of the moon, and with an elevation of seven and three-fourths feet. At Hell Gate this tidal wave is met by that which entered at Sandy Hook, and which, owing to the configuration of the channel, traveled more slowly. The meeting and overlapping of these two tides cause differences between the harbor and the Sound, which produces the violent currents of the East River. If a partition could be placed across Hell Gate, the difference in the range of the tide on its sides would sometimes amount to ten feet. As matters stand, a difference of one foot in the level is often observed off Hallett's Point, within the space of forty yards. The westerly current through Hell Gate is commonly called the "ebb stream," since it joins the ebb-stream of New York Harbor.

Yearly tide tables are published by the United States Coast Survey, which give the computed times of high water for the most important ports of the United States, and constants to apply to these times to obtain the time of high water at neighboring places. By simply applying these constants in the manner directed in the table, the time of high water at any place can be determined easily.

WINDS.

Wind is air in motion. The direction of the wind is designated by the point of the compass from which it blows. All winds are caused directly or indirectly by changes of temperature. If two neighboring regions become very unequal in temperature from any cause, the air of the warmer region being lighter than the other will ascend and be poured over it from above, while the heavier air of the colder region will flow in below to supply its place. Thus a difference in the temperature of the two districts gives rise to two currents of air-one blowing from the colder to the warmer along the surface of the earth, and the other from the warmer to the colder in the From the works of Lieutenant Charles H. Judd, U.S.N., one of the highest authorities on the subject.

upper regions of the atmosphere, and these currents will continue to flow until the equilibrium is restored.

Changes of temperature, although the prime cause of all winds, bring about changes of barometic pressure and changes of the specific gravity of the air, both of which are commonly considered as causes of wind; they are, however, but secondary causes.

The rotation of the earth would alone produce no permanent wind, because if there were no other disturbing causes the atmosphere would, by friction upon the earth's surface, soon acquire the same velocity of rotation as that of the portion of the earth upon which it rested, but the earth's rotation materially modifies the operation of other disturbing causes. Since the earth is nearly a spherical body, rotating upon its axis once in twentyfour hours, the velocity of rotation of different parallels is very different.

If a mass of quiescent air from the parallel of 30° could be suddenly transported to the parallel of 15°, it would have an easterly motion of 103 miles per hour less than that of the parallel arrived at, that is, it would have a relative motion westward of 103 miles per hour. So, also, if a mass of air from the parallel of 15 could be suddenly transported to the parallel of 30 it would have an easterly motion of 103 miles per hour greater than that of the parallel arrived at. That is, in general, if air is transported from the equator toward the poles, it will have a relative motion eastward, and if air is transferred from a higher latitude toward the equator, it will have a relative motion westward.

Winds are classified as constant, periodical, and variable winds.

TRADES-CONSTANT WINDS.

When the portion of the earth's surface which is heated is a whole zone, as in the case of the tropics, a surface wind will get in towards the equator from both sides, these having united will ascend, and then separating, flow