396
by the sum of six annual precessions. And the like is to be saidof the polar distances and right ascensions of other stars.
Now the fact is that, after the completion of the aboveprocess, the differences of the polar distances and right ascen-sions of stars are not found to be exactly accounted for by thequantity and law of precession : in some stars the differences aregreater than what they ought to be by the effect of precession, 1,1others less. And this fact being ascertained, our attention > sdrawn towards the mode by which the quantity of precession I sascertained.
f
The precession, or the retrogradation of the intersection 0the equator and ecliptic on the ecliptic, is not a phenomenon 0immediate observation. It requires, in all cases, some slig' )tcomputation ; which computation may be made either from th echanges it produces on the right ascensions of stars, or from th echanges in north polar distances, or from the differences oflongitudes of stars computed, for different epochs *, and fr° nlthe respective values of the right ascensions and polar distance*of stars belonging to those epochs (see Chapter VIII). No"'whichever be the method used, the mean quantity of the p re 'cession is that which results from a great number of stars, three orfour hundred, for instance; and even if there were any undeteefeinequality, equally affecting, however, all the stars, yet, s ,ncethe effect of such inequality would be blended with that of P r ^cession, the quantity of the precession (or what is so deem eobtained from all the stars, ought to agree with the mean P recession deduced from numerous observations of any one star 'But, if we suppose any peculiar movements to belong to a,1 5 rone, or to more stars, such peculiar movements would affect tb®quantity of precession determined by the preceding method, a 'vitiate it. Reversely, if the mean quantity of the precessi 011deduced from the comparison of three hundred stars should dift erfrom the quantity resulting from the comparison of fifty l 0 "® 1tudes of the star Arcturus , for instance, it would infallibly foil 0 " 1
* See pp. 184 , &c. of this Work. M. Zach computed the longit 11 ^of thirty-five principal stars, in order to determine the precession.