their return in different places. They are affected by straits and shoals, islands and continents, channels, winds, and a variety of other causes, so that many hours intervene between the time of high water at one place and at another on the same coast. These may be considered exceptions to the general principles in the preceding theory.

The following remarks of Mr. Ferguson may be added, as worthy of much consideration: It is not to be doubted, but that the Earth's quick rotation 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 in a month for, otherwise, the tides would be more unequal in their heights and times of their return, than we find they are. But however the Earth's rotation may bring the poles of its axis and those of the tides together, or how far the preceding tides may affect those which follow, 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 by theory." withstanding the justness of these observations, it was thought every student of philosophic mind would wish to know the theory of the tides, as regulated by the influence of the great heavenly bodies.

Not

The air, being a fluid, and extending much higher than the water, must be more affected by the unequal attraction of the Sun and Moon. Surrounding the whole Earth, and moving without obstruction, it must have tides more extensive, and generally higher, than those of the ocean.

The tides are of vast utility. They benefit us in agriculture, and assist us in navigation. The agitation they give to the water, together with saltness, prevents the ocean from becoming a vast reservoir of contagion and death. What infinite wisdom and goodness are displayed in giving such inconceivable power of benefiting us to bodies immensely distant!

What are the tides? How do we discover that there are tides? Who first discovered the true cause of the tides? Who wrote so amply on the tides as to make the theory of them in a measure his own? Who first discovered the true cause of the tide on the side of the Earth opposite to the Moon? If all parts of the Earth were equally attracted by the heavenly bodies, would there be tides? How is it that attraction causes a tide on the side of the Earth opposite to the Moon? Will the effect be the same, if the Earth be drawn away from the surface of the water, as if the water were drawn up on the land? How far is the circle of low water from the points of highest elevation in the tides? How far does this circle extend, and how does it move? How have some accounted for the tide on the side of the Earth opposite to the Moon? How do the tides travel? What declination have they? In what time does every place below the polar circles have two tides ? When the Moon is in the equator, to what does the circle of low water extend? How must every place then have its return of tides? When does the circle of low water recede from the poles? How many tides occur at the poles in a revolution of the Moon? At the poles, how long is it between a tide and a succeeding tide? Where have places but one tide in a revolution from the Moon round to the Moon again? Where is the Moon, when the tides return at equal intervals in all latitudes? When the Moon is in any degree of declination, how do places distant from the equator, but below the circle of low water, have their return of tides? When does a place in north declination have a higher tide on the side of the Earth opposite to the Moon, than on the side next to her? Why is the point of highest elevation not directly under, but after the Moon? What other occurrences are similar to this? If the Sun attract the Earth more than the Moon, why does it not raise a higher tide? When does the influence of the Sun cause the tides to be earlier, and when later, than they would be by the attraction of the Moon? What are spring tides? When do they happen? What are neap tides, and when do they occur? Why are the tides happening at the change and full about the equinoxes higher than those of other seasons? At what time of year do the highest tides known happen? Why have small seas, unconnected with the ocean and lakes, no perceptible tides? What prevent the regular return of the tides, according to the motions of the Moon? What are Mr. Ferguson's remarks respecting the regularity of the tides? Have we reason to suppose there are tides in the air? Why may they be more extensive and high than those of the ocean? In what are the tides beneficial to us?

CHAPTER VIII.

Eclipses.

AN eclipse is a partial or total obscuration of a heavenly body.

These

So far as astronomical observation has extended, the Sun is the only heavenly luminary in the solar system, that shines by its own light. The planets are in themselves opaque, and shine only by reflecting the solar rays. Hence on the side of these not illuminated by the Sun, dark shadows are cast. shadows are in the form of vast cones extending into the heavens. They are but privations of light in the space hid from the Sun. That they are not coextensive with the Sun's light, but terminate at a distance far more limited, is evident, because the primary planets never eclipse each other. Mars, though often in opposition to the Sun, is never eclipsed by the Earth's shadow. This must therefore terminate before it reaches that planet. (Plate vi. Fig. 8.) Let S be the Sun, A E the Earth, À BE the Earth's dark shadow, terminating at B. From this figure it is evident, that, when a luminous body is larger than a dark body, intercepting its rays, and causing a shadow, such shadow must end at the point where the rays from the extremes of the luminous body cross each other beyond the dark body; and that, as the Sun is far larger than the planets of our system, their shadows must terminate at points beyond the planets opposite to the Sun, at the intersection of the solar rays. The primary planets eclipse their secondaries, and the secondaries their primaries. The Earth's shadow eclipses the Moon; the Moon's shadow the Earth. But, when the Earth is eclipsed by the Moon, the Sun is darkened to

some of the inhabitants of the Earth. Hence eclipses of the Earth are usually denominated eclipses of the Sun.

The shadow of the Earth, when longest, is about 219 of its semi-diameters. Different computations make a trifling difference in the mean extent of this shadow. If the diameters of the Earth and Sun be taken as before stated, and the shadow be computed from these, it will be found to be about (217) semi-diameters of the Earth; equal to 864,094 miles,

If the Moon evolved in the plane of the ecliptic, an eclipse would happen at every conjunction and opposition, or at every change and full. But, her orbit being inclined to that circle in an angle of 5° 9′ 3′′, varying a little at different times, eclipses cannot happen except when she is in or about her nodes. In every other part of her orbit, she is either too far north or south to eclipse the Sun, or to fall into the Earth's shadow and be herself eclipsed. Plate vi. Fig. 11. represents the number of digits eclipsed up to 12 on the right hand, where the eclipse, being at the node, is total. The limit is different in different species of eclipses. For if the Moon be within about 170 of either of her nodes at the change, there will be a solar eclipse. But lunar eclipses can happen but when she is within about 11° of her nodes. The greatest limit in solar eclipses, according to the tables in the author's larger work, is 18° 11', the least, 16° 28'; the greatest in lunar, 11° 51', the least, 10° 11'.

In lunar eclipses, when a part only of the Moon's disk is covered, the eclipse is denominated partial; when the whole disk is (covered, total (when the centre of the disk passes through the centre of the shadow, central. (Plate vi. Fig. 10.)

The Moon is visible, when totally immersed in the Earth's shadow, appearing of a dusky red color, like burnished copper. It is probable, that the refracted rays of the Sun cause this phenomenon. These, trav

ersing the atmosphere of the Earth, are by it turned inward, so as to fall on the Moon, and render her distinctly to be seen.

In a lunar eclipse, all to whom the Moon is visible, see her in the same instant of absolute time.

Solar eclipses are much more frequent than lunar; but most of the former are invisible at any particular part of the Earth.

The dark shadow of the Moon sometimes reaches to the Earth, eclipsing a small portion of its surface; sometimes that dark shadow is terminated before it arrives at the Earth. In the latter case, the Sun, at the centre of an eclipse, appears like a luminous ring. The eclipse is then called annular. (Plate v. Fig. 5.) This beautiful phenomenon was seen in some parts of New England on the morning of April 3, 1791; at Washington, September 17, 1811; and in the eastern parts of the United States, February 12th of the year 1831. The dark shadow of the Moon is longest, when she is in perigee and the Earth in aphelion; shortest, when she is in apogee, and the Earth in perihelion. The inhabitants of our republic have had the satisfaction of viewing two annular eclipses, since the commencement of the present century; one, September 17, 1811, the other, February 12, 1831. cording to computation, they will have the pleasure of seeing another, September 18, 1838; the annular eclipses being three for the century.

Ac

Two total solar eclipses are computed for the United States during the century; one, June 16, 1806, the other, August 7, 1869. It will appear from this, and from inspection of the tables of the semi-diameters of the Sun and Moon, that annular eclipses of the Sun are more frequent than total eclipses of the same luminary.

The Moon's partial shadow is called her penumbra. All the inhabitants over whom this shadow extends, see the Sun partially eclipsed. In Plate vi. Fig. 10.