AUGUST, 1820.

MY readers are aware, that eclipses of the Sun are of two kinds, either partial, or total. Total, when the Sun's disc is entirely covered by the Moon; partial, when only a part of the Sun's disc is covered by the Moon. This part may be of every possible dimensions, a very small portion only being obscured by the Moon in its passage over the Sun, or so great a part may be obscured as to leave but a very small part of the Sun's disc visible. The magnitude of the part obscured depends on the relative proportions of the apparent diameters of the Sun and Moon, and the direction of the Moon's path by the Sun.

The partial eclipses are divided into two kinds: those, which are annular, or not annular. Annular are those so called, from the body of the Moon wholly covering a part of the body of

the Sun in such a manner, that the luminous part of the Sun appears in the form of a ring round the dark orb of the Moon: The breadth of this ring varies every moment, from the time, that the Moon's orb, being totally immersed in that of the Sun, leaves its circumference on one side, till its opposite side touches the opposite side of the Sun's disc. The breadth of the ring is constantly increasing on the one side, from the smallest conceivable speck, till the breadth on both sides is the same; and this equality can be but momentary, as the Moon continues its course, making now the breadth of the ring on the first side greater than that on the second, till at last the ring disappears in the same manner as it began to appear. The ring is perfect to those, from whose eye a line drawn through the centre of the Moon passes through the centre of the Sun: to all others the breadth of the ring will be unequal, as will be evident to any one by an easy experiment.

Make two circles, the one of any diameter you please, the other with a diameter shorter than that of the first. The first circle may be drawn on a large piece of paper, the other is to be drawn on another piece of paper, and cut out

exactly, and made black on one side. Now let the first circle represent the Sun, and through the centre of it draw a line to represent a portion of the ecliptick, and through the centre draw another line, making a small acute angle with it. With a pin put through the centre of the black circle, let it move along this second line; and, as it enters into the white circle, representing the Sun, you will see, by the gradual diminution of the white part, how the luminous part of the Sun gradually diminishes, and the obscure part increases. When the whole of the black circle has entered the other, you will see the form of the visible orb of the Sun the moment before the ring is formed: as the black circle moves on, you will see how the ring of light varies in its dimensions; and when the pin passes through the centre of the white circle, you will perceive, that the breadth of the ring is the same all round. This will be only momentary; for, as the black circle proceeds, the thickness of the ring on the side towards which it is moving diminishes, that on the other increasing, till the edge of the black circle touches the edge of the white circle, and the ring altogether disappears.

This little experiment shows you the state of the Sun and Moon, when the ring is at one time perfect. Now, instead of drawing the second line through the centre of the white circle, take a point at a little distance from the centre in the first line, and draw your second line through that point, and move the black circle in the direction of that line; and you will, as before, mark during the progress the formation of the white ring around it, but it will never be of the same uniform thickness all round.

By varying the situation of the point taken in the diameter of the white circle, you will perceive, what a variety of forms the annular eclipses are capable of; and you may pursue the experiment, by extending the diameter of the white circle, and taking a point in the line thus extended without the circle, and draw through it the line marking the path of the Moon's centre over the disc. As before, make the pin through the black circle move in this line, and every form of eclipse may be made clear to your conceptions, and in a better manner perhaps than can be expressed by words. For you may draw the line of the Moon's path either above

or below the diameter drawn through the white circle; and this will show you the cases, when the Moon enters the Sun's disc on the northern or southern side. The extreme cases will be seen; the Moon just touches the Sun, or enters into its disc, making only a partial eclipse.

The case of a total eclipse may be explained in the same manner. But for this purpose the circle, representing the Moon, must have the same or a greater diameter than that representing the Sun. As before, you will make the black circle pass over the other, and you will then readily perceive that there cannot be a total eclipse, if the apparent diameters are the same, unless, in the passage of the black over the white circle, the centres at one time coincide, and then the total eclipse will be momentary, and it must be a central eclipse. When an eclipse is to be represented, and it is total for some little time, the diameter of the black circle must be made larger than that of the white circle, and you will perceive, from that experiment, how short a time will elapse, even in a central eclipse, during which the white circle is completely covered by the black, or there is a total eclipse of the Sun.