« PoprzedniaDalej »
pagans for offering sacrifices to their idols, and upbraiding the Emperor Maxentius, to his face, with the most flagrant acts of tyranny and oppression. She was condemned to suffer death by rolling a wheel over her body stuck round with iron spikes.
27.-ADVENT SUNDAY. This and the three subsequent Sundays which precede the grand festival of Christmas take their name from the Latin advenire, to come into; or from the word adventus, an approach.
30.-SAINT ANDREW. Andrew was the son of James, a fisherman at Bethsaida, and younger brother of Peter. He was condemned to be crucified on a cross of the form of an X; and, that his death might be more lingering, he was fastened with cords. The Order of the Thistle is described in our last volume, p. 283. Astronomical Occurrences
In NOVEMBER 1819. The Sun enters Sagittarius at 36 m. past 2 in the morning of the 230; and he rises and sets during this month as stated in the following
TABLE Of the Sun's Rising and Setting for every fifth Day. November 1st, Sun rises 12 m. after 7. Sets 48 m. past 4
Equation of Time. From the apparent time, as indicated by a good sun-dial, subtract the following quantities, and the remainders will be the mean time corresponding to these several epochs.
7 7 7 7 7
37 44 51
TABLE. Monday, Nov. 1st, from the time by the dial subtract 16 15 Saturday, 6th,
16 14 Thursday, 11th,
15 52 Tuesday, 16th,
15 8 Sunday, 21st,
14 3 Friday, 26th,
9th, 19 11 night.
5 afternoon. First Quarter, - 24th,
1 Moon's Passage over the Meridian. The Moon will pass the first meridian at the following times in the course of this month; which will afford convenient opportunities for observation, if the weather prove favourable: viz. November 9th, at 53 m. after 5 in the morning. 10th, 38
6 11th, 20
4 in the evening.
10 Phase of Venus.
11.94161 digits. Dark part
0.65839 Eclipses of Jupiter's Satellites. The following are the eclipses of Jupiter's first and second satellites, which will be visible at the Royal Observatory this month: viz.
1st Satellite 6th day, at 21 m. after 6 evening.
6 2d Satellite 7th,
Other Phenomena. Jupiter will be in quadrature at 45 m. after 10 in the evening of the 1st of this month. The Moon will be in conjunction with ß in Taurus at 7 m. after 8 in the morning of the 5th ; with Pollux at 53 m. after 6 in the evening of the 7th; with Spica in Virgo at 3 m. after 6 in the morning of the 15th; and with a in Scorpio at 30 m. past 3 in the afternoon of the 18th. On the EFFECTS of GRAVITATION.
[Continued from p. 263.) Among the effects of gravitation on terrestrial bodies, there is none more astonishing, or more demonstrative of its power, than the periodic flux and reflux of the ocean; nor, amongst all the inquiries which relate to these effects, is there one more interesting than that which relates to the Tides. The weight of bodies; their descent to the surface of the Earth when unsupported; their deflection from the direction of their projectile motions, when these are not perpendicular to the surface of the Earth; and the vibrations of the pendulum, are so many striking proofs of the universal influence of terrestrial gravity ; while the variations in these at different points of the Earth's surface afford strong indications of its figure, and even furnish a ground-work for the application of science, by which we are enabled to attain conclusions relative to the ratio of the equatorial and polar diameters. But the phenomena of the tides afford the nearest and most striking proof we possess of the gravitation of terrestrial matter towards the celestial bodies.
The tides are produced by the action of the heavenly bodies, particularly the Sun and Moon, upon the waters of the ocean. The varieties in the relative situations and motions of these bodies, and the in clination of their orbits compared with the plane of the equator, produce inequalities in the tides, which modern investigation has been able to trace to their corresponding causes, and thus to apply these irregularities themselves to the establishment of the general theory; and prove that the tides, as well as the inequalities to which they are subject, are the necessary effects of universal gravitation.
If the waters of the sea covered the whole surface of the Earth to an equal depth, and experienced the action of gravity only, without any disturbing influence from the motion of the Earth, they would take a spherical form: but, on account of the rotary motion of the Earth, they ought to assume the form of an ellipsoid of revolution, the axis of the Earth being its minor axis. If the force of gravity combined with the centrifugal force, arising from the diurnal rotation of the Earth, were the only forces which acted upon the waters of the ocean, the ellipsoid would finally attain a state of stability, and its form would, in consequence, undergo no farther alteration. But, as the sea is also subject to the action of the Sun, it gravitates towards that body, each of its molecules being attracted in proportion to its distance from that luminary: supposing the Sun to be in the plane of the equator, this plane would then no longer retain its circular form, but become elliptic; the parts immediately beneath the Sun being raised above the other by his direct influence upon them, while those on the opposite side, being less attracted than the intermediate parts, would yield more freely to the influence of the centrifugal force resulting from the rotation of the Earth, and form the opposite part of the ellipse, which a section of the Earth through the equator would now exhibit. From this it follows, that there would result from the action of the Sun alone, two high and two low tides, the high tides being in the line of the Sun's action, or 180° from each other, and the low tides at right angles to this line, or at 90° from the first. If the Earth and the Sun had con
stantly the same angular velocity, or moved together, the solar action would always be exerted upon the same point; and the relative positions of the elevated and depressed parts of the ocean, with respect to the other parts, would then be constant; but, as the Sun continually changes his position with respect to the Earth, in his apparent progress from east to west, the elevations and depressions of the waters which arise from the influence of his attraction, combined with the rotary motion of the Earth, experience the same change, and follow his motions. The two high tides, therefore, happen at the time of the Sun's pas. sage over the meridian of that place, and over the opposite meridian, and the true low tides at the meri. dians that are 90° distant from these; and as the Sun passes over every point of the equator in the course of 24 hours, the interval between the two high tides would therefore be 12 hours, and the time of low water 6 hours distance from these. If the Sun remained constantly in the plane of the equator, as has been supposed, the waters would always be highest at the equator, and diminish gradually from thence to the poles, where there would be no elevation; but, as the Sun is sometimes on one side of the equator and sometimes on the other, the most elevated points of the ocean will follow the same course, and consequently be affected by his declination.
The Sun, however, is not the only body which acts upon the waters of the ocean, and gives rise to the phenomena of the tides; the Moon also produces her effect; and by supposing her to act alone, as has been done with respect to the Sun, the same reasoning may be applied to her effects as has been employed relative to the action of that luminary. If the action of these two bodies were independent of cach other, there would be four high and four low tides every 24 hours, except when their actions were both exerted in the same direction, that is, when the lumi. naries were either in conjunction or opposition. But,