the eclipse was therefore partial, not central, and the number of digits indicates to what extent of surface the eclipse advanced.

Calculations have been made as to the longest time that a solar eclipse can be either total, or annular, under the most favourable circumstances; and it has been found that 12 minutes 24 seconds is the longest time of an annular eclipse, and 7 minutes 58 seconds the longest time of a total eclipse.

The periods which the Earth and Moon take to perform their respective revolutions, and the degree of obliquity between their orbits, have enabled astronomers to calculate how many eclipses may occur in a year. There must be two eclipses of the Sun in every year, and the number may be as high as five, but it cannot be greater. No solar eclipse is visible in every part of the portion of the Earth which is turned towards the Sun, because the shadow of the Moon is too narrow to take in the whole diameter of the Earth at once, as may be seen in the preceding figures. A solar eclipse cannot be total at the same moment to a portion of the Earth more than 180 miles in diameter; but it may be partial at the same moment to a portion of the Earth 4900 miles in diameter. We may further observe that a solar eclipse does not happen at the same point of time at all places where it is perceived: it appears earlier to those persons situated towards the west, than to those situated towards the east, because the motion of the Moon and of her shadow is from west to east. Moreover, the extent of solar obscuration is different at different latitudes on the Earth's surface according as we may suppose a spectator to be more or less distant from the line which connects the three bodies in question.

K 2

LUNAR ECLIPSES.

As the solar eclipse was owing to the opaque body of the Moon hiding the light of the Sun from the Earth, and took place when the Moon was new, or in conjunction; so the lunar eclipse is due to the opaque body of the Earth hindering the solar rays from reaching the Moon, and takes place when the Moon is full, or in opposition.

In the following figure, s represents the Sun, E the Earth, m the Moon in opposition. The Sun, shining upon the Earth, causes a shadow to be thrown behind, which has the form of a cone; that is, it tapers to a point like a sugar-loaf. The reason why the shadow thus tapers to a point is, that the Sun is so much larger than the Earth. The distance of this point, or the total length of the Earth's shadow, is about 840,000 miles, when the Earth is nearest to the Sun; and about 870,000 miles, when she is at her furthest distance therefrom: it follows, therefore, that if any body come behind the Earth, in a right line with the Sun and the Earth, and not at a greater distance from the Earth than has just been named, that body will be immersed in the Earth's shadow.

This is the case with the Moon: we have seen that she is, on an average, about 240,000 miles distant from the Earth; and that she revolves round the Earth in a plane nearly coincident with the plane of the Earth's orbit. In our figure the shadow of the Earth is represented as having a conical tendency; if now the Moon m were beyond the apex of this cone, she would altogether escape from the Earth's shadow, but, as part