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Western Time: Tides

These high tides are called Spring Tides, and the low ones Neap Tides. The close relation which the times of high water bear to the times of the moon's meridian passage, shows that the moon's influence in raising the tides is much greater than that of the sun. While the whole attraction of the sun upon the earth far exceeds that of the moon, yet, owing to the greater proximity of the latter, the difference between its attraction at the centre of the earth and at the nearest or most remote point of its surface (which difference produces the tides) is about two and a half times as great as the difference of the sun's attraction at the same points. Though each of these bodies may be supposed to cause two tidal waves the tides must be regarded as the result of their combined action.

At the time of full and change of the moon, the combined effect produces the spring tides, and high water is then higher and low water lower than at mean tides. When the moon is in perigee, or nearest the earth, the rise and fall is sensibly increased. When the moon is in quadrature, or 90º degrees from the sun, the attraction of the two bodies upon the waters act in opposition, and the Neap Tides are produced.

Very small tides will take place about the time of the earth's perihelion passage, if the moon is in apogee and also in quadrature. During the first and third quarters of the lunar month the solar wave lies to the west of the lunar wave, and the combined tide wave will be to the westward of that, due to the moon alone; and this causes an acceleration of the time of high water commonly called priming. In the second and fourth quarters the sun's influence acts to retard the lunar wave, and cause what is known as lagging of the tides. The interval of 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 Diurnal Inequality is a regular change, considerable in amount, and almost universal in prevalence. This change depends principally upon the moon's being north or south of the equator; its maximum is consequent on, but not always simultaneous with the moon's greatest declination, and the period of its vanishing corresponds in like manner with the moon's passing the equator. If the declination of the moon is of the same name as the latitude of the place, the greater of the daily tides occurs next after the upper transit of the moon; but if the latitude and declination have contrary names, the higher tide of the day follows the lower culmination of the moon. The Diurnal Inequality sometimes affects the time of high water as much as two hours, that of low water about forty minutes; at the same time a variation of a foot may be observed in the height of high water, and that of three feet in that of low water. Such effects are too great to be neglected, either in the prediction of tides or the reduction of soundings. 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. How the Tides are produced by the Sun and Moon. Each of these bodies excites by its attraction upon the waters of the sea, two gigantic waves, which flow in the same direction round the world as the attracting bodies themselves apparently do. The two waves of the moon, on account of her greater nearness, are about 3½ times as large as those excited by the sun. One of these waves has its crest on the quarter of the earth's surface which is turned toward the moon, the other is at the opposite side. Both these quarters possess the flow of the tide, while the regions which lie between have the ebb. Although in the open sea the height of TIDES. the tide amounts to only about 3 ft., and even in certain narrow channels, where the water is squeezed together, rises to 30 ft., the might of the phenomenon is nevertheless manifest from the calculation of Bessel, according to which a quarter of the earth covered by the sea possesses during the flow of the tide about 25,000 cubic miles of water more than during the ebb, and that, therefore, such a mass of water must in 674 hours flow from one quarter of the earth to the other. NOTES.

A wave 100 ft. in breadth and 100 ft. in depth travels about 15 miles an hour; one 1000 ft. broad and 1000 ft. deep about 48 miles per hour; one 10,000 ft, deep and 10,000 ft. broad about 154 miles per hour. During heavy gales the waves of the Atlantic are from 24 to 36 ft. high, half above and half below the mean level of the sea. This increases to 45 ft. in a raging storm, being about 600 ft. from crest to crest, and moving 30 miles an hour.

The average height of clouds above the earth is between I and 2 miles, but highly electrified ones are much lower. Lightning clouds are seldom more than 700 yards from the ground, and often they are much closer. Some clouds are abour 20 sq. miles in surface, and above a mile in thickness, while others are only a few yards or inches.

Cirrus cloud consists of streaks, wisps, and fibres, vulgarly called "mare's tails," which may increase in any or all directions. Of all clouds it has the least density, the greatest elevation, and the greatest variety of extent and direction, or figure. It remains for a short time when found in the lower parts of the atmosphere and near other clouds, and longest when alone in the sky and at a great height. When streaks of cirrus run quite across the sky in the direction in which a light wind happens to blow, the wind will probably soon blow hard, but remain steady. When the fine threads of the cirrus appear blown or brushed backward at one end, as if by a wind prevailing in these elevated regions, the wind on the surface will ultimately veer round to that point. Cumulus, a cloud in dense convex heaps in rounded forms definately terminated above: the lower surface remains roughly horizontal. When of moderate height and size, of well-defined curved outline, and appearing only during the heat of the day, they indicate a continuance of fair weather. But when they increase with great rapidity, sink down to the lower parts of the atmosphere, and do not disappear toward evening, rain may be expected.

Stratus is a continuous extended sheet of cloud, increasing from below upward. It is the lowest sort of cloud. It generally forms about sunset, grows denser during the night, and disappears about sunrise.

Cirro-Cumulus is composed of well-defined, small, rounded masses, lying near each other, and quite separated by intervals of sky. It is commonly known as a "mackerel sky:" it occurs frequently in summer, and is attendant on warm and dry weather.

Cirro-Stratus. This cloud partakes partly of the characteristics of the cirrus and stratus. In distinguishing it, attention must be paid, not so much to the form, which is very variable, but to the structure, which is dense in the middle and thin toward the edges. It is a precursor of storms, and from its greater or less abundance and permanence, it gives some indication at the time when the storm may be expected.

Cumulo-Stratus. This cloud is formed by the cirro-stratus blending with the cumulus, either among its piled-up heaps, or spreading underneath its base as a horizontal layer of vapor Cumulo-Cirro-Stratus or Nimbus. This is the rain-cloud. At a considerable height a sheet of cirro-stratus cloud is spread out, under which cumulus clouds drift from windward; these rapidly increasing, unite at all points, forming one continuous mass, from which rain falls.

Scud. When a rain-cloud is seen approaching at a distance, cirri appear to shoot out from its top in all directions, and it has been observed that the more copious the rainfall, the greater is the number of the cirri thrown out from the cloud. Rainbows. "A rainbow in the morning- Sailors take warning; A rainbow at night Is the sailor's delight."

Morning rainbows indicate the advance of rain-cloud from the west when it is clear in the east; and the fall of rain at the time of day when the temperature should be rising, is regarded as a prognostic of a change to wet, stormy weather. On the contrary, the conditions under which a rainbow can appear in the evening are: the passing of the rain-cloud to the east, and a clearing up in the west at the time of day when the temperature has begun to fall, thus indicating a change from wet to dry weather. "The evening gray and the morning red, put on your hat, or you'll wet your head." This does not refer to a high red dawn, which may be regarded as a prognostic of settled weather. But if clouds be red and lowering later in the morning, it may be accepted as a sign of rain. The Barometer-What is it. The construction of a mercurial barometer is as follows:-A glass tube, about 33 inches in length, open at one end, is filled with mercury, and while the unsealed end is covered, is inverted into a basin of mercury. As soon as the cover is removed the mercury in the tube will flow out until it stands about 30 inches above the mercury in the basin. At that point the pressure of the air upon the liquid in the basin is equal to the pressure of the liquid in the tube (the space in the upper part of the tube being a vacuum), and the flow ceases. When the air becomes heavier its pressure upon the basin will be greater and force the mercury in the tube higher; if the air becomes lighter, the mercury in the tube will sink. The barometer, therefore, shows directly only the weight of the atmosphere, but thereby indirectly the future state of the weather is indicated. The barometer, feeling the pressure of the air, shows at once when that pressure is changing. If the pressure at one place on the earth's surface be greater than at another, the air has a tendency to move from the place where the pressure is greater, toward that where it is less, and thus wind is caused A change of weather comes almost always with a change of wind, and the extent of this change of weather depends on the fact of the new wind being warmer or colder, damper or drier, than that which has been blowing. Any conclusions drawn from its movements must be checked by observations of temperature, moisture of the air, present direction and force of wind, and state of the sky, before any correct opinion can be formed as to what may be expected. In general, whenever the level of the mercury continues steady, settled weather may be expected; but when it is unsteady, a change must be looked for, and perhaps a gale. A sudden rise of the barometer is very nearly as bad a sign, as a sudden fall, because it shows that atmospherical equilibrium is unsteady. In an ordinary gale the wind often blows hardest when the barometer is just beginning to rise, directly after having been very low. Besides these rules for the instrument, there is a rule about the way in which the wind changes which is very important. It is well-known to every sailor, and is contained in the following couplet: "When the wind shifts against the sun, trust it not, for back it will run." The wind usually shifts with the sun, i. e., from left to right in the Northern Hemisphere. A change in this direction is called veering. Thus an east wind shifts to west, through south-east; south, south-west; and a west wind shifts to east through north-west, north, and north-east. If the wind shifts the opposite way, viz., from west to south-west, south, and south-east, the change is called backing, and it seldom occurs, unless when the weather is unsettled. However, slight changes of wind do not follow this rule exactly; for instance, the wind often shifts from south-west to south and back again. In the Southern Hemisphere the motion with the sun is, of course, from right to left, and, therefore, the above rules will necessarily be reversed.

The Thermometer. A thermometer is graduated thus: It is placed in snow, or pounded ice, and a mark is made opposite the end of the column of mercury.

This is the freezing-point. It is next surrounded with the steam of boiling-water, and a second mark made to show the boiling-point. The space between these two points is then divided into a number of equal parts, which varies with the description of thermometer. One hundred and eighty is the number in the Fahrenheit (the one in general use), and as the freezing-point is called 32, the boilingpoint becomes 212. In the centigrade, which is used for scientific purposes freezing-point is marked o, and boiling-point 100; and in Reaumur's instrument these points are o and 80 respectively. As the barometer shows weight and pressure of the air, so the thermometer shows heat and cold, or temperature. The result of many observations show that in the Northern Hemisphere the thermometer rises with east, south-east, and south winds; with a south-west wind it ceases to rise and begins to fall; it falls with west, south-west, and south winds; and with a north-east wind it ceases to fall and begins to rise. And in the Southern Hemisphere the thermometer rises with east, north-east, and north winds; with a northwest wind it ceases to rise and begins to fall; it falls with west, south-west, and south winds, and with a south-east wind it ceases to fall and begins to rise. Besides the use of the thermometer, in conjunction with the barometer, in foretelling the changes of weather by its aid, information may frequently be derived for sailors when passing from one ocean current to another;

it also may give warning of the vicinity of ice.