reigns added together amount to six hundred and forty-eight years; which, being divided equally among the thirty kings, give to every one a reign of twenty-one years and a half very near.  Sixty-three kings of France have sat upon the throne; these have, one with another, reigned about twenty years each.  This is the usual course of Nature.  The ancients, therefore, were mistaken when they supposed the durations in general of reigns to equal that of generations.  They, therefore, allowed too great a number of years, and consequently some years must be subtracted from their computation.

Astronomical observations seem to have lent a still greater assistance to our philosopher.  He appears to us stronger when he fights upon his own ground.

You know that the earth, besides its annual motion which carries it round the sun from west to east in the space of a year, has also a singular revolution which was quite unknown till within these late years.  Its poles have a very slow retrograde motion from east to west, whence it happens that their position every day does not correspond exactly with the same point of the heavens.  This difference, which is so insensible in a year, becomes pretty considerable in time; and in threescore and twelve years the difference is found to be of one degree, that is to say, the three hundred and sixtieth part of the circumference of the whole heaven.  Thus after seventy-two years the colure of the vernal equinox which passed through a fixed star, corresponds with another fixed star.  Hence it is that the sun, instead of being in that part of the heavens in which the Ram was situated in the time of Hipparchus, is found to correspond with that part of the heavens in which the Bull was situated; and the Twins are placed where the Bull then stood.  All the signs have changed their situation, and yet we still retain the same manner of speaking as the ancients did.  In this age we say that the sun is in the Ram in the spring, from the same principle of condescension that we say that the sun turns round.

Hipparchus was the first among the Greeks who observed some change in the constellations with regard to the equinoxes, or rather who learnt it from the Egyptians.  Philosophers ascribed this motion to the stars; for in those ages people were far from imagining such a revolution in the earth, which was supposed to be immovable in every respect.  They therefore created a heaven in which they fixed the several stars, and gave this heaven a particular motion by which it was carried towards the east, whilst that all the stars seemed to perform their diurnal revolution from east to west.  To this error they added a second of much greater consequence, by imagining that the pretended heaven of the fixed stars advanced one degree eastward every hundred years.  In this manner they were no less mistaken in their astronomical calculation than in their system of natural philosophy.  As for instance, an astronomer in that age would have