for high water on the side of the Earth opposite to the Moon, in the following manner: As the Earth and Moon move around their common centre of gravity, that part of the Earth which is at any time turned from the Moon, being about 7000 les farther from the centre of gravity, than the side next the Moon, would have a greater centrifugal force than the side next her. At the Earth's centre, the centrifugal force will balance the attractive force; therefore as much water is thrown off by the centrifugal force on the side which is turned from the Moon, as is raised on the side next her by her attraction. From the universal law, that the force of gravity diminishes as the square of the distance increases, it results, that the attractive power of the Moon decreases in intensity at every step of the descent from the zenith to the nadir; and consequently that the waters on the zenith, being more attracted by the Moon than the Earth is at its centre, move faster towards the Moon than the Earth's centre does: And as the centre of the Earth moves faster towards the Moon than the waters about the nadir do, the waters will be, as it were, left behind, and thus, with respect to the centre, they will be raised. The reason why the Earth and waters of our globe do not seem to be af fected equally by the Moon's attraction, is, that the earthy substance of the globe, being firmly united. does not yield to any difference of the Moon's at tractive force; insomuch that its upper and lower surface must move equally fast towards the Moon; whereas the waters, cobering together but very lightly, yield to the different degrees of the Moon's attractive force, at different distances from her. The length of a lunar day, that is, of the interval from one meridian passage of the Moon to another, being, at a mean rate, 24 hours, 48 minutes and 44 seconds, the interval between the flux and the reflux of the sea is not, at a mean rate, precisely six hours, but twelve minutes and eleven seconds more, so that the time of high water does not happen at the same hour, but is about 49 minutes later every day. The Earth revolves on its axis in about twenty-four hours; if the Moon, therefore, were stationary, the same part of our globe would return beneath it, and there would be two tides every twenty-four hours; but while the Earth is turning once upon its axis, the Moon has gone forward 13° in her orbitwhich takes forty-nine minutes more before the same meridian is brought again directly under the Moon. And hence every succeeding day the time of high water will be fortynine minutes later than the preceding, For example:-Suppose at any place it be high water at 3 o'clock in the afternoon, upon the day of new Moon; the following day it will be high water about 49 ininutes after 3; the day after, about 38 minutes after 4; and so on, How is this phenomenon otherwise explained, by the laws of gravity, merely? Are the Earth and waters of the globe affected equally, by the Moon's attraction? Why not? What is the average interval between the flux and reflux of the sea? What is the length of a lunar day, and of the interval of the flux and reflux of the sea? How is this daily etardation of the tides accounted for? Give an example? till the next new Moon. The exact daily mean retardation of the tides is thus determined: The mean motion of the Moon, in a solar day, is 130.17639639 Now, as 150 is to 60 minutes, so is 120.19074917 to 48′ 44′′. It is obvious that the attraction of the Sun must produce upon the waters of the ocean a like effect to that of the Moon, though in a less degree; for the great mass of the Sun is more than compensated by its immense distance. Nevertheless, its effect is considerable, and it can be shown, that the height of the solar tide is to the height of the lunar tide as 2 to 5. Hence the tides, though constant, are not equal. They are greatest when the Moon is in conjunction with, or in opposition to, the Sun, and least when in quad rature. For in the former case, the Sun and Moon set together, and the tide will equal the sum of the solar and lunar tides, and in the latter they act against each other, and the tide will be the difference. The former are called Spring Tides; the latter, Neap Tides. The spring tides are highest, when the Sun and Moon are near the equator, and the Moon at her least distance from the Earth. The neap tides are lowest, when the Moon in her first and second quarters is at her greatest distance from the Earth. The general theory of the tides is this: When the Moon is nearest the Earth, her attraction is strongest, and the tides are the highest; when she is farthest from the Earth, her attraction is least, and the tides are the lowest. From the above theory, it might be supposed that the tides would be the highest when the Moon was on the meridian. But it is found that in open seas, where the water flows freely, the Moon has generally passed the north or south meridian about three hours, when it is high water. The reason is, that the force by which the Moon raises the tide continues to act, and consequently the waters continue to rise, after she has passed the meridian. For the same reason, the highest tides, which are produced by the conjunction and opposition of the Sun and Moon, do not happen on the days of the full and change; neither do the lowest tides happen on the days of their quadratures.—But the greatest spring tides commonly hap Are the tides uniformly high? When, and on what account do they differ? What are these extreme tides called? When are the spring tides highest? When are the neap tides lowest? What is the general theory upon this subject? Does it necessarily result from this theory, that the tide is highest when the Moon is on the meridian? What reason is assigned for this? What similar fact is accounted for upon the same principle? pen 14 days after the new and full Moons; and the least neap tides 12 days after the first and third quarters. The Sun and Moon, by reason of the elliptical form of their orbits, are al ternately nearer to and farther from the Earth, than their mean distances. In consequence of this, the efficacy of the Sun will fluctuate between the extremes 19 and 21, taking 20 for its mean value, and between 43 and 59 for that of the Moon. Taking into account this cause of difference, the highest spring tide will be to the lowest neap as 59+21 is to 43-19, or as 80 to 24, or 10 to 3. The relative mean influence is as 51 to 20, or as 5 to 2, nearly.— Herschel's Astr. p. 339. Though the tides, in open seas, are at the highest about three hours after the Moon has passed the meridian, yet the waters in their passage through shoals and channels, and by striking against capes and headlands, are so retarded that, to different places, the tides happen at all distances of the Moon from the meridian; consequently at all hours of the lunar day. In small collections of water, the Moon acts at the same time on every part; diminishing the gravity of the whole mass. On this account there are no sensible tides in lakes, they being generally so small that when the Moon is vertical, it attracts every part alike; and by rendering all the waters equally light, no part of them can be raised higher than another. The Mediterranean and Baltic Seas have very small elevations, partly for this reason, and partly because the inlets by which they communicate with the ocean are so narrow, that they cannot, in so short a time, either receive or discharge enough, sensibly to raise or sink their surfaces. Of all the causes of difference in the height of tides at different places, by far the greatest is local situation. In wide-mouthed rivers, opening in the direction of the stream of the tides, and whose channels are growing gradually narrower, the water is accumulated by the contracting banks, until in some instances it rises to the height of 20, 30, and even 50 feet. Air being lighter than water, and the surface of the atmosphere being nearer to the Moon than the surface of the sea, it cannot be doubted but that the Moon raises much higher tides in the atmosphere than in the sea. According to Sir John Herschel these tides are, by very delicate observations, rendered not only sensible, but measurable. Upon the supposition that the waters on the surface of the Moon are of What is the comparative force of the solar and lunar attraction upon the Earth? To what is owing the great difference in the time of high water at places lying under the same meridian? Why are there no tides upon lakes, and small collections of water? To what cause, more than to all others, is the different height of tides owing? Explain this. Is it probable that the Moon exerts any influence of attraction on the atmosphere? Why is it probable ? Are the atmospheric tides sufficiently sensible to be appreciated? the same specific gravity as our own, we might easily determine the neighs to which the Earth would raise a lunar tide, by the known principle, that the attraction of one of these bodies on the other's surface is directly as its quantity of matter, and inversely as its diameter. By making the calculation, we shall find the attractive power of the Earth upon the Moon to be 21.777 times greater than that of the Moon upon the Earth. CHAPTER XXIII. THE SEASONS-DIFFERENT LENGTHS OF THE DAYS AND NIGHTS. The vicissitudes of the seasons and the unequal lengths of the days and nights, are occasioned by the annual revolution of the Earth around the Sun, with its axis inclined to the plane of its orbit. The temperature of any part of the Earth's surface depends mainly, if not entirely, upon its exposure to the Sun's rays. Whenever the Sun is above the horizon of any place, that place is receiving heat; when the Sun is below the horizon it is parting with it, by a process which is called radiation. The quantities of heat thus received and imparted in the course of the year, must balance each other at every place, or the equilibrium of temperature would not be supported. Whenever, then, the Sun remains more than twelve hours above the horizon of any place, and less beneath, the general temperature of that place will be above the mean state; when the reverse takes place, the temperature, for the same reason, will be below the mean state. Now the continuance of the Sun above the horizon of any place, depends entirely upon his declination, or altitude at noon. About the 20th of March, when the Sun is in the vernal equinox, and consequently has no declination, he rises at six in the morning and sets at six in the evening; the day and night are then equal, and as the Sun continues as long above our horizon as below it, his influence must be nearly the same at the same latitudes, in both hemispheres. From the 20th of March to the 21st of June, the days grow longer, and the nights shorter, in the northern hemisphere the temperature increases, and we pass from spring to mid-summer; while the reverse of this takes place in the How much grecter is the attractive power of the Earth upon the Moon, than that of the Moon upon the Earth? What occasions the vicissitudes of the seasons, and the unequal lengths of the days and nights? Upon what does the temperature at different places depend? Under what circumstances do the same places change their temperature? Are the quantities of heat, received and imparted, every year, always equal at the same places } Why is it so? When is the temperature of a place above, and when is it below its mean state? Upon what does the continuance of the Sun above the horizon of any place, depend? When is the Sun as long above our horizon as below it? During what season of the year is the temperature increasing? southern hemisphere. From the 21st of June to the 23d of September, the days and nights again approach to equality, and the excess of temperature in the northern hemisphere above the mean state, grows less, as also its defect in the southern; so that, when the Sun arrives at the autumnal equinox, the mean temperature is again restored. From the 23d of September until the 21st of December, our nights grow longer and the days shorter, and the cold increases as before it diminished, while we pass from autumn to midwinter, in the northern hemisphere, and the inhabitants of the southern hemisphere from spring to mid-summer. From the 21st of December to the 20th of March, the cold relaxes as the days grow longer, and we pass from the dreariness of winter to the mildness of spring, when the seasons are completed, and the mean temperature is again restored. The same vicissitudes transpire, at the same time, in the southern hemisphere, but in a contrary order.—Thus are produced the four seasons of the year. But I have stated not the only, nor, perhaps, the most efficient cause in producing the heat of summer and the cold of winter. If, to the inhabitants of the equator, the Sun were to remain 16 hours below their horizon, and only 8 hours above it, for every day of the year, it is certain they would never experience the rigours of our winter; since it can be demonstrated, that as much heat falls upon the same area from a vertical Sun in 8 hours, as would fall from him. at an angle of 60°, in 16 hours. Now as the Sun's rays fall most obliquely when the days are shortest, and most directly when the days are longest, these two causes, namely, the duration and intensity of the solar heat, together, produce the temperature of the different seasons. The reason why we have not the hottest temperature when the days are longest, and the coldest temperature when the days are shortest, but in each case about a month afterwards, appears to be, that a body once heated, does not grow cold instantaneously, but gradually, and so of the contrary. Hence, as long as more heat comes from the Sun by day than is lost by night, the heat will increase, and vice versa. What, at the same time, takes place in regard to the temperature, in the southern hemisphere? During what portion of the year is the temperature decreasing? For what reason? During what portion of the year is the cold increasing? Why is it so? What change of seasons, then, takes place, in the northern and southern hemisphere? What other changes complete the seasons of the year? Whence is it evident that the unequal lengths of the days and nights are not the only, nor perhaps the most efficient cause of the heat of summer, and the cold of winter? What two causes produce the greatest vicissitudes of heat and cold? Why, then, do we not have the hottest weather when the days are longest, and the contrary} |