nal. The Sea being thus put in motion, would continue to ebb and flow for several times, though the Sun and Moon should be annihilated, or their influence cease. When the Moon is in the equator, the Tides are equally high in both parts of the lunar day, or time of the Moons' revolving from the meridian to the meridian again; which is 24 hours and 50 minutes. But, as the Moon declines from the equator towards either pole, the Tides are alternately higher and lower at places having north or south latitude. One of the highest elevations, (which is that under the Moon,) follows her towards the pole to which she is nearest, and the other declines towards the opposite pole; each elevation describing parallels as far distant from the equator on opposite sides, as the Moon declines from it to either side, and consequently, the parallels described by these elevations of the water, are twice as many degrees from each other, as the Moon is from the equator; increasing their distance as the Moon increases her declination, till it be at the greatest; when the said parallels are at a mean state 47 degrees asunder, and on that day, the Tides are most unequal in their heights. As the Moon, returns toward the equator,the parallels described by the opposite elevations approach towards each other, until the Moon comes to the equator, and then they coincide. As the Moon declines toward the opposite pole at equal distances, each elevation describes the same parallel in the other part of the lunar day, which its opposite elevations described before. While the Moon has north declination, the greatest Tides in the northern hemisphere, are, when she is above the horizon, and the reverse when her declination is south. Thus it appears, that as the Tides are governed by the Moon, they must turn on the axis of the Moon's orbit, which is inclined 23 degrees and 28 minutes to the earth's axis at a mean state, and therefore the poles of the Tides, must be so many degrees from the poles of the earth, or in opposite points of the polar circles, go ing around them in every revolution of the Moon from any meridian to the same again. It is not, however, to be doubted, but that the quick rotation of the earth on his axis, brings the poles of the Tides nearer to the poles of the world, than they would be, if the earth were at rest, and the Moon revolved about it only once a month, otherwise the Tides would be more unequal in their heights, and times of their returns, than we find they are. But how near the earth's rotation may bring the poles of its axis, and those of the Tides together, or how far the preceding Tides may effect those that follow, so as to make them keep up nearly to the same heights and times of ebbing and flowing, is a problem more fit to be solved by observation than theory. In open Seas, the Tides rise but to very small heights in proportion to what they do in broad Rivers, whose waters empty in the direction of the stream of Tile:For, in channels growing narrower gradually, the water is accumulated by the opposition of the contracting bank. The Tides are so retarded in their passage through different shoals and channels, and otherwiseso variously affected by striking against Capes and Headlands, that to different places, they happen at all distances of the Moon from the meridian, and consequently at all hours of the lunar day. * Air being lighter than water, and the surface of the atmosphere nearer to the Moon, than the surface of the Sea; it cannot be doubted that the Moon raises much higher Tides in the air, than in the Sea. * In a register of the barometer kept for 30 years, the Professor Toaldo of Padua, added together all the heights of the mercury, when the Moon was in syzigy, when she was in quadrature, and when she was in the apogeal and perigeal points of her orbit. The apogeal exceeded the perigeal heights by 14 inches, and the heights in syzigy exceeded those in quadrature by 11 inches. The difference in these heights, is sufficiently great to show that the air is accumulated and compressed by the attraction of the Moon. Interrogations for Section Tenth. By whom was the cause of the TIDES first discovered? How does he explain it ? Who improved the idea of Kepler ? By what does he consider the waters to be attracted ? Why are the waters on the side of the earth next to the Moon, more attracted than the central parts ? Why are the central parts more attracted, than the waters on the opposite side? From what source is this explanation deduced? By what power is the earth constantly falling towards the Moon, and the Moon towards the earth? If this be actually the case, why do they not come together? Is it the centre of the earth that describes the annual orbit round the Sun ? Where is the centre of gravity between the earth and Moon? How much more matter does the earth contain, than the Moon? What is the centre of gravity between the two bodies? How is it found? Which has the greatest influence in raising Tides, the Sun or Moon? Are the Tides at the highest when the Moon is due north, or south? What is the reason? Do the Tides always answer to the same distance of the Moon from the meridian at the same places? Does the Moon approach nearer, and recede farther from the earth in each of her revolutions? At what time does she attract the earth most? At what time does she attract it the least? In what position are the Sun and Moon when the highest Tides are raised? What are Spring-Tides ? What are Neap-Tides? In what manner do the attractions of the Sun and Moon act on each other, to produce Spring-Tides? In what manner to produce Neap-Tides? Where is the moon when the Tides are equally high in both parts of the lunar day? What is understood by the lunar day ? · At what time are the Tides most unequal? In which hemisphere are the highest Tides, when the moon has north declination? Which when in her south declination? Do the Tides rise very high in open Seas? What are aerial Tides ? How were they discovered, and by whom? |