It may safely be pronounced, that no book of its age presents itself to us at this day, more completely free from exploded opinions in the physical sciences. With the exception perhaps of occasional allusions to "animal spirits," as a moving influence in the human body, the author's careful sifting sceptical mind seems, without having practically tested them, to have turned away from whatever doctrines were afterwards destined to fall before the test of experiment and induction. It was not that he was so much of a natural philosopher himself as to be able to test their truth or falsehood, but that with a wholesome jealousy, characteristic of the mind in which the Disquisition on Miracles was working itself into shape, he avoided them as things neither coming within the scope of his own analysis, nor bearing the marks of having been satisfactorily established by those whose more peculiar province it was to investigate their claims to be believed. At a later date, his friend D'Alembert admitted judicial astrology and alchemy as branches of natural philosophy in his "Systême Figuré des Connoissances Humaines." Cudworth, and even the scrutinizing Locke, dealt gravely with matters doomed afterwards to be ranked among popular superstitions, and Sir Thomas Browne, in some respects a sceptic, eloquently defended more "vulgar errors" than he exposed. Hobbes was, in the midst of the darkest scepticism, a practical believer in the actual presence of the spirits of the air; and Johnson, whose name, however, it may scarcely be fair to class in this list, as he did not profess, except for conversational triumph, to be a reducer and demolisher of unfounded beliefs, along with his partial admission of the existence of spectres, has left behind him many dogmatic announcements of physical doctrines, which the progress of science has now long buried under its newer systems. It is by no means maintained that Hume was beyond his age--or even on a par with its scientific ornaments, in physical knowledge; but merely that he showed a judicious caution in distinguishing, in his published work, those parts of physical philosophy which had been admitted within the bounds of true and permanent science, from those which were still in a state of mere hypothesis. His knowledge of physical science was probably not very extensive. A small portion of a collection of his notes on subjects that attracted his attention bear on this subject. The collection from which they are taken will be noticed in the next chapter; but as those which are set apart from the others, and are headed "Natural Philosophy," seem to have been written at an earlier period than the rest of the collection, and are appropriate to the present subject, they are here given. It is not expected that they will awaken in the natural philosopher any great respect for the extent of Hume's inquiries in this department of knowledge. NATURAL PHILOSOPHY. "A ship sails always swiftest when her sides yield a little.