the time of sun-setting and sun-rising, at any particular place, it will be visible there and invisible to the inhabitants of the opposite hemisphere, who have the sun above their horizon at that time; for the sun and moon are in opposite parts of the heavens at the time of a lunar eclipse. And with respect to solar eclipses, it is evident that they can only be seen at any place when the sun is above the horizon of that place. There is, however, a difference with regard to the visibility of a solar and lunar eclipse; for an eclipse of the moon has the same appearance to all the inhabitants of that hemisphere to which the moon is visible at the time, owing, in part, to the small dis-' tance of the moon from the earth. But an eclipse of the sun may be visible to some places and invisible to others in the same hemisphere of the earth, because the moon's shadow is small in comparison of the earth; for its breadth, excluding the penumbra, is only about 180 miles even in central eclipses.* Hence those places which are considerably distant from the path of the shadow will either have no eclipse at all, or a very small one; while places near the middle of the shadow will have the greatest possible. There is also a difference in the absolute time at which a solar eclipse happens at the various places where it is visible; for it appears more early to the western parts, and later to the eastern, on account of the motion of the moon (and of course her shadow) from west to east.

*

In most solar eclipses the moon's disc may be observed by a telescope to be covered by a faint light, which is attributed to the reflexion of light from the illuminated part of the earth. When the eclipses are total, the moon's limb is surrounded by a pale circle of light, which some astronomers consider as an indication of a lunar atmosphere, but others, as occasioned by the atmosphere of the sun; because it has been observed to move equally with the sun and not with the moon.

Dr. Halley, in describing a central eclipse of the sun, which happened at London in April, 1715, says, that although the disc of the sun was wholly covered by the moon, a luminous ring of a faint pearly light surrounded the body of the moon the whole time; and its breadth was nearly a tenth of the moon's diameter.

In lunar eclipses, the moon seldom disappears entirely; and on some occasions, even the spots may be distinguished through the shade; but this can only be the case when the moon is at her greatest distance from the earth at the time of the eclipse, for the nearer the moon is to the earth the darkness is the greater. In some instances, the moon has disappeared entirely; and the celebrated astronomer Heraclius, has taken notice of one where the moon could not be seen even with a telescope, though the night was remarkably clear.

Although eclipses of the sun and moon were long considered by the ignorant and superstitious as presages of evil, yet they are of the greatest use in astronomy, and may be employed to improve some of

* A penumbra is the faint shadow produced by an opaque body when opposed to a luminous one.

the most important and useful of the sciences. By eclipses of the moon the earth is proved to be of a globular form, the sun to be greater than the earth, and the earth greater than the moon. When they are similar in all their circumstances, and happen at considerable intervals of time, they also serve to ascertain the real period of the moon's motion. In geography, eclipses are of considerable use in determining the longitude of places, and particularly eclipses of the moon, because they are oftener visible than those of the sun, and the same eclipse is of equal magnitude and duration at all places where it is seen. In chronology, both solar and lunar eclipses serve to determine exactly the time of any past event.

We, in the dark eclipse, with filial awe
Trace the all-gracious Parent of the spheres ;
Their distances and their proportion learn;
Extending navigation; securing

The mariner thro' the tremendous waves.

EUDOSIA.

For the purpose of finding the longitude at places on the earth, eclipses of Jupiter's satellites are found much more useful than eclipses of the moon; not only on account of their happening more frequently, but on account of their instantaneous commencement and termination.

When Jupiter and any of his satellites are in a line with the sun, and Jupiter between the satellite and the sun, it disappears, being then eclipsed, or involved in his shadow. When the satellite goes behind the body of Jupiter, with respect to a spectator on the earth, it is said to be occulted, being hid from our sight by his body, whether in his shadow or not. And when the satellite comes into a position between Jupiter and the sun, it casts a shadow on the face of that planet, which is seen by a spectator on the earth as an obscure round spot. Lastly, when the satellite is in a line with Jupiter and the earth, it appears on his dise as a round black spot, which is termed a transit of the satellite.

*

As these phenomena appear at the same moment of absolute time at all places on the earth to which Jupiter is then visible, but at different hours of relative time, according to the distance between the meridian of the places at which observations are made, it follows that this difference of time converted into degrees will be the difference of longitude between those places. Suppose, for example, that a person at London observed an eclipse to begin at 11 o'clock in the evening, and that a person at Barbadoes observed the same at 7 o'clock in the evening, it is certain the eclipse was seen by both persons at the same moment of absolute time, although there is four hours' difference in their manner of reckoning that time: and this converted into degrees (at the rate of 15 degrees to an hour) is the

Absolute time is that which is computed from the same moment; relative is that which is computed from different moments.

difference of longitude between these two places-therefore Barbadoes is 60 degrees west from London, the time not being so far advanced there as at London.

Another phenomenon, somewhat similar to an eclipse, sometimes takes place, by which the longitude of places may be determined, although not quite so easily, nor perhaps so accurately, as by the eclipses of Jupiter's satellites. This is the hiding or obscuring of a fixed star or planet by the moon or other planet, which takes place when the moon or planet is in conjunction with the star. Appear ances of this kind are termed occultations. They are very little attended to except by practical astronomers, who employ them for the correction of the lunar tables, and settling the longitude of places, as already stated.

ON LIGHT.

Fairest of beings! first created light!

Prime cause of beauty! for from thee alone
The sparkling gem, the vegetable race,

The nobler worlds that live and breathe their charms,
The lovely hues peculiar to each tribe,

From thy unfading source of splendour draw

In thy pure shine, with transport I survey
This firmament, and these her rolling worlds
Their magnitudes and motions.

MALLET.

The nature of Light has been the subject of speculation and conjecture among philosophers, from the first dawnings of philosophy to the present day. But of all the conjectures which have been advanced on this curious and interesting subject, there is scarcely one supported by evidence sufficient to entitle it to preference over the other. There are, however, two opinions on this subject which have prevailed more generally than any of the others, and therefore, it may be proper to notice them here, although the design of the present work is rather to state what is known respecting any phenomenon, than to indulge in conjectures concerning it.

The celebrated Huygens considered light as a subtle fluid filling space, and rendering bodies visible by the undulations into which it was thrown. According to this theory, when the sun rises it agitates this fluid, the undulations gradually extend themselves, and at last striking against our eyes, we see the sun. This opinion of Hyugens was adopted by Euler, one of the best mathematicians that ever lived, who exerted the whole of his consummate mathematical skill in its defence.

Sir I. Newton and many other distinguished philosophers consider light as a substance consisting of small particles, constantly separating from luminous bodies, moving in straight lines, and rendering other bodies luminous by passing from them and entering the eye. Newton has been at great pains to establish this theory, and has certainly

shown that all the phenomena of light may be mathematically deduced

from it.

While Huygens and Euler have attempted to support their hypothesis, rather by starting objections to Newton's, than by bringing forward direct proofs, Newton and his disciples, on the contrary, have shown that the known phenomena of light are inconsistent with the undulations of a fluid, and that on such a supposition there can be no such thing as darkness at all. They have also brought forward a great number of direct arguments in support of their theory, which it has been impossible to answer.* But without giving a decided preference to any theory, we shall proceed to state some of its properties.

Roemer, a Danish astronomer, while engaged in making observations on the satellites of Jupiter, found that in eclipses they emerged from the shadow at certain times a few minutes later, and at others a few minutes sooner than they ought to have done according to the tables, which had been previously constructed to shew the times of their revolutions, eclipses, &c. On comparing these apparent irregularities together, he found that the eclipses happened before or after the computed time, according as the earth was nearer to or farther from Jupiter. Hence he formed the ingenious conjecture, which was soon demonstrated to be the case, that the motion of light is not instantaneous, as was then generally believed, but that it required a certain portion of time, to pass from the luminous body to the eye of the observer. According to Roemer's calculation, it was about seven minutes in traversing the radius of the earth's orbit; but it has since been found, that when the earth is exactly between Jupiter and the sun, his satellites are eclipsed about 8 minutes sooner than the time found by the tables; but when the earth is nearly in the opposite point these eclipses happen about 8 minutes later than that determined by the tables. It is therefore concluded that light takes up about 16 minutes of time to pass over a space equal to the diameter of the earth's orbit, which is at least one hundred and ninety millions of miles; it therefore moves at the rate of nearly 200,000 miles per second, which is about 10,300 times faster than the earth in its orbit, and 1,550,000 times quicker than a cannon ball.†

Behold the light emitted from the sun!

What more familiar, and what more unknown?
While by its spreading radiance it reveals
All Nature's face, it still itself conceals.
See how each morn it does its beams display,
And on its golden wings bring back the day!

* M. Delaval maintains, that all light is reflected by white particles, and coloured in its transmission. No transparent medium reflects any light when examined within a blackened bottle; this is shewn by experiments on 68 kinds of fluids, and on many kinds of glasses, and other substances. For this, and for the colours of the sea, M. Delaval proposes a very singular theory; but those who wish to become particularly acquainted with it, must consult his work on the permanent colours of opaque bodies.

+ The real time which light takes to pass from the sun to the earth is 8 minute 13 seconds.

How soon the effulgent emanations fly
Thro' the blue gulf of interposing sky!
How soon their lustre all the region fills,
Smiles on the valleys, and adorns the hills:
Millions of miles, so rapid is their race,
To cheer the earth, they in few minutes pass.
Amazing progress! At its greatest stretch,
What human mind can this swift motion reach?

BLACKMORE.

The velocity of light being known, it is easy to know the time it requires to arrive at the earth from any of the planets, or even the fixed stars if their distance be known. For it has been ascertained, that the reflected light of the planets and satellites, travels with the same velocity as the direct light of the sun or fixed stars; and that the velocity is the same from whatever distance it comes.

The discovery of Roemer has been completely confirmed by another most important discovery made by our countryman Dr. Bradley, while engaged in making a series of observations, with a view to determine the annual parallax of the fixed stars. This celebrated astronomer found that the aberration or difference between the true and apparent place of a fixed star, is occasioned by the progressive motion of light, combined with the motion of the earth in its orbit; and that this aberration when greatest amounted to 20232. Now the earth describes an arc of 20"-232, in 8′ 13′′, the time that light takes to pass over the semidiameter of the earth's orbit. This circumstance, therefore, not only affords one of the most convincing proofs of the motion of the earth in its orbit, but entirely overthrows both the Ptolemaic and Tychonic systems, and completely establishes the motion of the earth.

As the rays of light are known to proceed only in straight lines from luminous bodies, and as the earth is constantly moving forward in its orbit, it is evident that a ray of light proceeding from any celestial body will impinge on the earth at a different point from what it would have done had the earth been stationary. It is therefore necessary, in making astronomical observations with nicety, to make allowance for the aberration. When a fixed star or planet, for example, is seen through a tube or telescope, the tube does not point exactly to the true place of the star or planet, but to its apparent place, which is always more advanced in the direction we are moving than its true place, by a quantity equal to the aberration of the object. But this will, perhaps, be better understood by the following illustration, which is given by M. Maupertius in his Elements of Geography.

The direction," says he, "in which a gun must be pointed to strike a bird in its flight, is not exactly that of the bird, but of a point a little before it, in the path of its flight; and that so much the more as the flight of the bird is more rapid, with respect to the flight

* The aberration not only affects the longitude of a star or planet, but also its atitude, declensions, and right ascension,