LET. X.

OF THE LATITUDE AND LONGITUDE. 167

of absolute time, will enable us to compare thetime as it is reckoned at those places, and, bythat means, to find the meridians under whichthey are situated.

The moment when the moon begins to beobscured, by entering into the earth’s shadow, iscalled the beginning of the eclipse; and themoment when she leaves the shadow, the end.And if both these be properly observed, and amean be taken between them, this mean time iscalled the middle of the eclipse. The moon issometimes entirely hid by the earth’s shadow,and is then quite invisible. This is a total eclipseof the moon; and, in this cafe, the momentwhen she begins to disappear, is called the be-ginning of total darkness; and the momentwhen she begins to appear again, the end. Butwhen only a part of the moon is obscured, theeclipse is called a partial one ; and we have thenonly to observe when it begins and ends. It isalso to be remarked, that these eclipses happenonly about the time of the full moon, and eventhen not often.

From these circumstances it will readily ap-pear, that if an eclipse of the moon be observedat any two places which lie under the fame me-ridian, the clocks of those places, supposing themto be perfectly true, will strew either the be-ginning or the end of the eclipse at the sametime; but if the two places lie under different -M 4 meridians,