Now,here,both the Egyptians and the Greeks made an erroneous computation.It is true,indeed,that,according to the usual course of Nature,three generations last about a hundred and twenty years;but three reigns are far from taking up so many.It is very evident that mankind in general live longer than kings are found to reign,so that an author who should write a history in which there were no dates fixed,and should know that nine kings had reigned over a nation;such an historian would commit a great error should he allow three hundred years to these nine monarchs.Every generation takes about thirty-six years;every reign is,one with the other,about twenty.Thirty kings of England have swayed the sceptre from William the Conqueror to George I.,the years of whose 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.
