Bradley saw that Romer's observation concerning the tithe that light takes to go from the sun to the earth gave a ready expression for the velocity of light compared with that of the earth. The proportion, however, which he assumed as best suited to his observations was somewhat different; it was that of 10313 to 1, which made the radius of the circle of aberration 20", and the transverse axis of the ellipse in every case, or the whole change of position, 40". It was the shorter axis which Bradley had actually observed in the case of 7 Draconis, that star being very near the solsti tial colure, so that its change's of declination and,of latitude are almost the same. In order to show the truth of his theory, he computed the aberration of different stars, and, on comparing the results with his observations, the coincidence appeared almost perfect, so that no doubt remained concerning the truth of the principle on which he bad founded his calculations. He did not explain the rules themselves: Clairaut published the first investigation of these in the Memoirs of the Ac' ademy of Sciences for 1737. Simpson also gave a demonstration of them in his Essays, published in 1740.
It has been remarked, that the velocity of light, as assumed by Bradley, did not • exactly agree with that which Romer had assigned; supposing the total amount of the aberration 40r, it gave the time that light takes to come from the sun to the earth 8' 13"; but it is proper to add, that since the time of this astronomer, the velocity of light deduced from the eclipses of Jupiter's satellites has been found actly the same.
It is remarkable thaf the phenomenon thus discovered by Bradley and Molyneux, when in search of the parallax of the fixed stars, is in reality as convincing a proof of the earth's motion in its orbit, as the discovery of that parallax would have been.. It seems, indeed, as satisfactory as any evidence that can be desired. One only regrets in reflecting on this discovery, that the phenomenon of the aberration was not foreseen, and that, after being predicted from theory, it had been ascertained from observation. As the matter stands, however, the discovery both of the fact and the theory is highly creditable to its author.
In the imperfect outline which I have now sketched of one of the most interesting periods in the history of human knowledge, much has been omitted, and many great characters passed over, lost, as it were, in the splendour of the two: great luminaries which marked this epocha. Newton and Leibnitz are so distinguished from the rest even of the scientific world, that we can only compare them with one another, though, in fact, no two intellectual characters, who both reached the highest degree of excel lence, were ever more dissimilar.
For the variety of his genius, and the extent of his research, Leibnitz is perhaps al together unrivalled. A lawyer, a historian, an antiquary, a poet, and a philologist,— a mathematician, a metaphysician, a theologian, and I will add a geologer, he has in all these characters produced works of great merit, and in some of them of the highest excellence. It is rare that original genius has so little of a peculiar direction, or is disposed to scatter its efforts over so wide a field. Though a man of great inven tive powers, he occupied much of his time in works of mere labour and erudition, where there was nothing to invent, and not much of importance to discover. Of
his inventive powers as a mathematician we have already spoken; as a metaphysi cian, his acuteness and depth are universally admitted ; but metaphysics is a science in which there are few discoveries to be made, and the man who searches in it for novelty, is more likely to find what is imaginary than what is real: The notion of the Monads, those unextended units, or simple essences, of which, according • to this philosopher, all things corporeal and spiritual, material or intellectual, are formed, will be readily allowed to have more in it of novelty than truth. The pre established harmony between the body and the mind, by which two substances in capable of acting on one another, are so nicely adjusted from the beginning, that their movements for ever correspond, is a system of which no argument can do more than prove the possibility. And, amid all the talent and acuteness with which these doctrines are supported, it seems to argue some unsoundness of under standing, to have thought that they could ever find a place among the established principles of human knowledge.
Newton did not aim at so wide a range. Fortunately for himself and for the world, his genius was more determined to a particular point, and its efforts were more con centrated. Their direction was to the accurate sciences, and they soon proved equally inventive in the pure and in the mixed mathematics. Newton knew how to transfer the truths of abstract science to the study of things actually existing, and, by returning in the opposite direction, to enrich the former by ideas derived from the latter. In expe rimental and inductive investigation, he was as great as in the pure mathematics, and his discoveries as distinguished in the one as in the other. In this double claim to re nown, Newton stands yet unrivalled; and though, in the pure mathematics, equals may perhaps be found, no one, I believe, will come forward as his rival both in that science and in the philosophy of nature. His caution in adopting general principles; his dislike to what was vague or obscure ; his rojootiou of oil thcurioo from which precise conclu sions cannot be deduced ; and his readiness to relinquish those that depart in any de gree from the truth, are, throughout, the characters of his philosophy, and distinguish it very essentially from the philosophy of Leibnitz. The characters now enumerated are most of them negative, but without the principles on which they are founded, invention can hardly be kept in the right course. The German philosopher was not furnished with them in the same degree as the English, and hence his great talents have run very frequently to waste.
It may be doubted also, whether Leibnitz's great metaphysical acuteness did not sometimes mislead him in the study of nature, by inclining him to those reasonings which proceed, or affect to proceed, continually from the cause to the effect. The attributes of the Deity were the axioms of his philosophy ; and he did not reflect that this foundation, excellent in itself, lies much too deep for a structure that is to be raised by so feeble an architect as man ; or, that an argument, which sets out with the most profound respect to the Supreme Being, usually terminates in the most unwar rantable presumption. His reasonings from first causes are always ingenious ; but