HISTORY OF PHYSICAL ASTRONOMY.

165

These observations have proved eminently serviceable in conducting astro-nomers to the true theory of the planet’s motion; for, as it takes eiglity-fouryears to effect a complete revolution round the sun, it is manifestly impos-sible to arrive at definite ideas respecting the irregularities of its motionexcept by means of observations which are considerably distant from eachother. In 1790, the observations of the planet had become sufficientlynumerous to induce the Academy of Sciences of Paris to propose the theoryof its motion as the subject of a prize. Delambre was on this occasion thesuccessful competitor. The elliptic elements were determined by a skilfulcombination of the observations subsequent to the discovery of the planet,and the perturbations occasioned by Jupiter and Saturn were computed bymeans of the formulae of the Mecanique Celeste . The theory of the planetestablished upon these principles was found to agree very well with threeancient positions which had been already detected among the observationsof Flamstead, Lemonnier, and Mayer. Delambre employed it in cal-culating tables of the planet, which continued for a few years to representthe actual motion with tolerable accuracy. As time, however, wore on,the discordance between theory and observation constantly increased, anda growing desire was felt by astronomers that the theory should be revisedand employed in the calculation of new tables. This task was undertakenby Bouvard, wliose tables of the planet appeared in 1821. While engagedin correcting the elements for this purpose, he experienced a singularcause of perplexity. He found that the earlier observations of Flamstead,Mayer, Lemonnier, and Bradley, might all be satisfied by an ellipse of adeterminate form and position; but he attempted in vain to include in thesame orbit the observations of the planet made subsequently to its discovery,although these, on the other hand, might also be satisfied by an ellipse, theelements of which were different from those of the other. “ The construc-tion of the tables, then,” says Bouvard, “involves this alternative:—ifwe combine the ancient observations with the modern, the former willbe sufficiently well represented, but the latter will not be so, with all theprecision which their superior accuracy demands; on the other hand, ifwe reject the ancient observations altogether, and retain only the modern,the resulting tables will faithfully conform to the modem observations, butwill very inadequately represent the more ancient. As it was necessaryto decide between these two courses, I have adopted the latter, on theground that it unites the greatest number of probabilities in favour oftruth, and I leave to the future the task of discovering whether thedifficulty of reconciling the two systems is connected with the ancientobservations, or whether it depends on some foreign and unperceived causewhich may have been acting upon the planet.”

Bouvard accordingly rejected the ancient observations, and constructedhis tables exclusively on the basis of those comprehended between the dis-

had been on one occasion determined by that astronomer. Burchardt, having made newresearches into Flamstead’s Historia Celestis, discovered four additional observations ofthe planet, one on the 2nd April, 1712, and the other three on the 4th, 5th, and 10th ofMarch, 1715. Bouvard having carefully inspected all Lemonnier’s observations foundten more positions of the planet. It is remarkable, that of six observations by thatastronomer, two were made by him on two consecutive days, and the other four on fourconsecutive days. If it had only occurred to him to compare together his observationson either of these occasions, he could not fail to have anticipated Herschel in the discoveryof the planet. Le Verrier, in his investigation of the theory of Uranus , rejects Flam-stead’s observation of the 5th March, 1712, and adopts another by the same astronomerdated the 29th April, 1712, without, however, mentioning the person who first detected it.