(3). Upon a reconsideration of Mr. Airy's Treatise On Tides and Waves, I am no longer disposed to say, as I have said vol. II. p. 311, that for the actual case of the distribution of land and water, nothing has been done to bring the hydrodynamical theory of oceanic tides into agreement with observation. In this admirable work, Mr. Airy has, by peculiar artifices, solved problems which come so near the actual cases that they may represent them. He has, in this way, deduced the laws of the semi-diurnal and the diurnal tide, and the other features of the tides which the equilibrium theory in some degree imitates; but he has also, taking into account the effect of friction, shown that the actual tide may be represented as the tide of an earlier epoch ;—that the relative mass of the moon and sun, as inferred from the tides, would depend upon the depth of the ocean (Art. 455);-with many other results remarkably explaining the observed phenomena. He has also shown that the relation of the cotidal lines to the tide waves really propagated is, in complex cases, very obscure, because different waves of different magnitudes, travelling in different directions, may coexist, and the cotidal line is the compound result of all these.

(4). Page 509. Mr. Airy's explanation of the phenomena termed by Sir D. Brewster a new property of light, is completed in the Philosophical Magazine for Nov. 1846. It is there shown that a dependence of the breadth of the bands upon the aperture of the pupil, which had been supposed to result from the theory, and which does not appear in the experiment, did really result from certain limited conditions of the hypothesis, which conditions do not belong to the experiment; and that when the problem is solved without those limitations, the discrepance of theory and observation vanishes so that, as Mr. Airy says, "this very remarkable experiment, which long appeared inexplicable, seems destined to give one of the strongest confirmations to the Undulatory Theory."

TRINITY COLLEGE, CAMBRIDGE,

November 7, 1846.

PREFACE TO THE FIRST EDITION.

Ar the present day, any endeavour to improve and extend the Philosophy of Science may hope to excite some interest. All persons of cultivated minds will agree, that a very important advantage would be gained, if any light could be thrown upon the modes of discovering truth, the powers that we possess for this end, and the points to which these may most profitably be applied. Most men, too, will allow, that in these respects much remains to be done. The attempts of this kind, made from time to time, are far from rendering future efforts superfluous. For example, the Great Reform of Philosophy and Method, in which Bacon so eloquently called upon men to unite their exertions in his day, has, even in ours, been very imperfectly carried into effect. And, even if his plan had been fully executed, it would now require to be pursued and extended. If Bacon had weighed well all that Science had achieved in his time, and had laid down a complete scheme of rules for scientific research, so far as they could be collected from the lights of that age, it would still be incumbent upon the philosophical world to augment as well as preserve the inheritance which he left; by combining with his doctrines such new views as the advances of later times cannot fail to produce or suggest; and by endeavouring to provide, for every kind of truth, methods of research as effective

VOL. I.

b

as those to which we owe the clearest and surest portions of our knowledge. Such a renovation and extension of the reform of philosophy appears to belong peculiarly to our own time. We may discern no few or doubtful presages of its approach; and an attempt to give form and connexion to the elements of such a scheme cannot now be considered premature.

The Novum Organon of Bacon was suitably ushered into the world by his Advancement of Learning; and any attempt to continue and extend his Reform of the Methods and Philosophy of Science may, like his, be most fitly preceded by, and founded upon, a comprehensive Survey of the existing state of human knowledge. The wish to contribute something, however little it may be, to such a Reform, gave rise to that study of the History of Science of which the present Work is the fruit. And the effect of these researches has been, a persuasion, that we need not despair of seeing, even in our own time, a renovation of sound philosophy, directed by the light which the History of Science sheds. Such a reform, when its Epoch shall arrive, will not be the work of any single writer, but the result of the intellectual tendencies of the age. He who is most forward in the work will wisely repeat the confession of his sagacious predecessor: Ipse certè (ut ingenue fatear) soleo æstimare hoc opus magis pro partu Temporis quàm Ingenii.

To such a work, whensoever and by whomsoever executed, I venture to hope that the present Volumes may be usefully subservient. But I trust, also, that

in its independent character, as a History, this book may be found not altogether unworthy of the aim which its title implies.

It is impossible not to see that the writer of such a history imposes upon himself a task of no ordinary difficulty and delicacy; since it is necessary for him to pronounce a judgment upon the characters and achievements of all the great physical philosophers of all ages, and in all sciences. But the assumption of this judicial position is so inevitably involved in the functions of the historian (whatever be his subject), that he cannot justly be deemed presumptuous on that account. It is true, that the historian of the progress of science is required by his undertaking to judge of the merits of men, in reference to subjects which demand a far intenser and more methodical study than the historian of practical life gives to the actions of which he treats; and the general voice of mankind,—which may often serve as a guide, because it rarely errs widely or permanently in its estimate of those who are prominent in public life,—is of little value when it speaks of things belonging to the region of exact science. But to balance these disadvantages, and to enable us to judge of the characters who must figure in our history, we may recollect that we have before us, not the record only of their actions, but the actions themselves; for the acts of a philosopher are his writings. We do not receive his exploits on tradition, but by sight; we do not read of him, we read him. And if I may speak of my own grounds of trust and encouragement in venturing on such a task, I knew that my life had