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must both be of the sun; for in

every year there

are at least two eclipses of the sun.

There can never be more than three lunar eclipses in a year; and in some years there are none at all.

Though the number of solar eclipses is greater than that of lunar, in the ratio of 3 to 2; yet more lunar than solar eclipses are visible in any particular place, because a lunar eclipse is visible to an entire hemisphere, while a solar eclipse is only visible to a part.

Central eclipses are comparatively rare phænomena; for though there are about 28 such eclipses in every cycle of 18 years and 10 days, yet the terrestrial space over which every one of them appears to be central, is a narrow belt, in some cases a mere mathematical line traced across the enlightened hemisphere of the earth.

In all eclipses, whether of the sun or moon, there are three points particularly to be observed, the beginning, the middle, and the end; and every precaution is taken to note the precise moment of each of these three phases. In total eclipses there are two other circumstances to be observed; these are the total immersion and the commencement of the emersion. In total eclipses, then, there are five particulars to be observed, the commencement of the immersion, which is the commencement of the eclipse; the total immersion; the middle of the eclipse; the com

mencement of the emersion; and the total emersion, which is the end of the eclipse.

There is also another circumstance to be observed in every eclipse, namely, its greatness or extent, that is, the portion of the luminary eclipsed, or which is covered by the shadow. To measure this, the diameters of the sun and moon are supposed to be divided into twelve equal parts, which are called digits; and an eclipse is said to be so many digits, according to the number of those parts which are obscured. In total eclipses of the moon, it is often said to be eclipsed more than twelve digits, though the diameter of the moon is only estimated at that number. The expression then implies that the earth's shadow covers more than the disc of the full moon, and the shadow is measured as if it were a part of the luminary eclipsed.

The satellites of Jupiter are very frequently eclipsed. They revolve very rapidly round the planet; their orbit is very little inclined to that of Jupiter; and the mass of each of them very small, compared with his. It happens, therefore, that at each revolution they are necessarily plunged in the shadow of the planet, in other words eclipsed. As it happens that these eclipses, from the great distance of Jupiter, can be perceived at different parts of the earth at the same instant, they afford a certain and very common method of ascertaining with exactness the differ

ence of the meridians of these different places, and consequently their relative longitudes. There are two points to be principally attended to in the eclipses of Jupiter's satellites; viz. the immersion of the satellite in the shadow, and its emersion. While the earth is at a certain distance from the point in which Jupiter is in opposition to the sun, but before that opposition, the spectator will see the immersion of the satellite into Jupiter's shadow; but if it be the first satellite, on account of its nearness to Jupiter, the emersion is never seen in this relative state of the several bodies, the satellite being behind Jupiter's body at the said emersion. The other three satellites may have both their immersions and emersions visible in the same eclipse; the possibility of this, however, depends upon the position of the earth. When the earth, after the opposition, reaches a certain point, the emersions of the first satellite become visible, while the immersions of that satellite are no longer to be seen: both immersions and emersions of the other three satellites may, in this case, be visible; in like manner as in the former position of the several bodies. Before the oppositions of Jupiter to the sun, the immersions and emersions happen to the west of Jupiter; after opposition they happen to the east. If an astronomical telescope be used, which reverses objects, the appearances will, of course, be just the contrary.

For many other particulars connected with the observation of this interesting class of phænomena, the reader must consult the larger and more scientific treatises of astronomy; especially those of Gregory, Woodhouse, Vince, and Delambre. We must now turn to other branches of philosophical inquiry.

LECTURE XXV.

CHEMISTRY.

GENERAL PRINCIPLES.

CHEMISTRY has been termed, with some propriety, the anatomy of matter: and its object is to discover the component parts of bodies, and, if necessary, to form them into new combinations. This last, indeed, is the principal occupation of the practical or trading chemist.

It is by the agency of heat and mixture, or, in other words, by the action of the particles of one body upon those of another, that the chemist is enabled to analyse or decompose the different substances which nature presents to his view. The matter of heat, or caloric, is the most powerful agent; for it has an attraction for most bodies, enters into their pores, and by its repulsive powers produces a decomposition of their constituent particles. There exists also in many cases a reciprocal attraction between the particles of one substance and those of another: this is seen particularly in all cases of solution; as when a lump of loaf sugar is put into a glass of water, it is said to be dissolved; that is, the particles of the sugar are intimately blended and united with those of the water by a mutual attraction. Here let it be observed that mere mechanical mixture is very

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