LET. XXIT.

SUN AND MOON,

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After the sun, moon, and nodes, however,have been once in a line of conjunction, theywill return so nearly to the same hate again in223 mean lunations, or about eighteen years andten days, as that the fame node, which was inconjunction with the fun and moon at the be-ginning of the first of these lunations, will bewithin less than half a degree of a line of con-

> j junction with the fun and moon again, when: the last of these lunations is completed. And,therefore, in that time, there will be a regular; period of eclipses, or returns of the fame eclipsesfor many ages. But the falling back of the line' of conjunction of the fun and moon, with re-spect to the line of the nodes, in every period,will at length exhaust it, and after that it willnot return again in less than 12,492 years.

; • Another period for comparing and examining■ eclipses, which happen after long intervals oftime, is that which consists of 6890 mean luna-tions, or about 557 years and 21 days; in whichtime the fun and node will meet again so nearly,1 is to be little more than eleven seconds distant:i but it will not be the same eclipse that returns,a as in the shorter period above-mentioned. Thesej I periods are said to have been discovered by theChaldeans six or seven hundred years before thebirth of Christ; but M. Badly, in his Histoirede l’Astronomie Ancienne, has endeavoured to’ fliew, that the invention is of a much earlier

C c 2 date.