phenomenon; and, after examining the case, came to the conclusion, that Nature certainly abhorred a vacuum, but for the first two-and-thirty feet only! It was his pupil, TORRICELLI, who first demonstrated the true cause of the phenomenon, by a most happily-imagined experiment. The water, rising as it does only to a certain height, must in fact, he remarked, be not drawn, but pushed up into the barrel of the pump; and it can only be so pushed by the pressure of the atmosphere on the exposed portion of it. The thirty-two feet of water in the body of the pump are merely a counterbalance to a column of air of equal basis, reaching to the top of the atmosphere. But, if so, it then occurred to him, another liquid, heavier or lighter than water, will, in similar circumstances, ascend to a correspondingly less or greater height, a less or greater quantity of it being, of course, required to balance the atmospheric column. Mercury, for example, is about thirteen times and a-half as heavy as water; it ought to mount, therefore, only to the height of about twenty-eight inches, instead of thirty-two feet. So, taking a glass tube of about three feet in length, and hermetically sealed (that is, made air-tight) at one end, he first filled it completely with mercury, and then closing it with his finger reversed it, and plunged it into a basin of the same liquid metal: when, withdrawing his finger, he had the gratification of seeing the liquid in the tube, now forming one body with that in the basin, descend, until, exactly as he had anticipated, there remained suspended a column of twenty-eight inches only. Well, by this experiment, in every way a most ingenious and beautiful one, Torricelli had in reality invented the instrument we now call the Barometer: and yet, strange to say, it was left to another to discover that he had done so. It was the great PASCAL, a man of sublime and universal genius, who, upon hearing of Torricelli's experiment, first made the remark, that the inference which he had deduced from it might, if true, be confirmed beyond the possibility of dispute, by carrying the mercurial tube to a considerable elevation above the earth, when, the atmospheric column being diminished, that of the mercury, which was supposed to be its balance, ought to be shortened likewise in a corresponding proportion. We had thus, therefore, a measure of the weight of the atmosphere in all circumstances, and consequently of the height of any position to which we could carry the instrument. The experiment was performed, and the result was what Pascal had anticipated. In this way, at length, was completed a discovery, the first steps towards which had been made two thousand years before during the whole of which period the phenomena best fitted to suggest it were matter of daily observation to every one; but which, nevertheless, at last escaped even several of the greatest philosophers who had made the nearest approaches to its development.

To return, however, for a moment to the subject of the happy appli

cation to philosophical purposes of common facts. This subject is the more worth our attention, as it opens a field of invention and discovery to which all men have, in one sense, equal access; although it is only the mind which has been rightly prepared, by previous knowledge and reflection, that is in a condition to profit by the opportunity. Another example which may be given is that of the discovery of the mode of engraving called Mezzotinto, if we are to accept the account which ascribes it to the famous PRINCE RUPERT. It is said to have been suggested to him by his observing a soldier one morning rubbing off from the barrel of his musket the rust which it had contracted from being exposed to the night dew. The prince perceived, on examination, that the dew had left on the surface of the steel a congeries of very minute holes, so as to form the resemblance of a dark engraving, parts of which had been here and there already rubbed away by the soldier. He immediately conceived the idea that it would be practicable to find a way of covering a plate of copper in the same manner with little holes, which being inked and laid upon paper, would undoubtedly produce a black impression; while, by cutting or scraping away, in different degrees, such parts of the surface as might be required, the paper would be left white wherever there were no holes. Pursuing this thought, he at last, after a variety of experiments, invented a species of steel roller, covered with points, or salient teeth, which, being pressed against the copperplate, indented it in the manner he wished; and then the roughness thus occasioned had only to be scraped down, where necessary, in order to produce any gradation of shade that might be desired.1

The celebrated modern invention of the Balloon is said to have had an origin still more simple. According to some authorities, the idea was first suggested to STEPHEN MONTGOLFIER, one of the two brothers to whom we owe the contrivance, by the waving of a linen shirt, which was hanging before the fire, in the warm and ascending air. Others tell us, that it was his brother JOSEPH who first thought of it, on perceiving the smoke ascending his chimney one day, during the memorable siege of Gibraltar, as he was sitting alone, and musing on the possibility of penetrating into the place, to which his attention had been called at the moment by a picture of it, on which he had accidentally cast his eyes. It is known, however, that the two brothers, who were paper-makers, and as such conversant with an apparently convenient material for their proposed experiment, had, before this, studied and made themselves familiar with Priestley's work on the different kinds of air; and it is even said that Stephen had conceived the idea of navigating

1 This is the account given by Vertue the engraver. But others maintain that mezzotinto scraping was the invention of Lieut.Col. de Siegen; that he thus engraved the portrait of the Landgravine of Hesse, in 1643;

and that Prince Rupert learnt the art of him, and carried it into England, where he much improved it. See Heinecken, Idée des Estampes, p. 208.

the heavens, by the employment of a gas lighter than common atmospheric air, on his way home from Montpelier, where he had purchased that book.1 Newton, also, is well known to have been indebted for the first hint of certain of his great optical discoveries to the child's amusement of blowing bubbles out of soap; and, as Dr. Pemberton has ingeniously observed, in his account of that great man's philosophy, "it is suitable to this mode of thinking that he has, in his Observations on Daniel,' made a very curious as well as useful remark, that our Saviour's precepts were all occasioned by some ordinary circumstance of things then especially before him."

Such is the way in which out of a very little matter has not unfrequently grown a large produce of philosophy. Originally, all human knowledge was nothing more than the knowledge of a comparatively small number of such simple facts as those from which Galileo deduced the use of the pendulum for the measurement of time, and Newton the explanation of the system of the heavens. All the rest of our knowledge, and these first rudiments of it also, a succession of individuals have gradually discovered, each his own portion, by their own efforts, and without having any teacher to instruct them. In other words, everything that is actually known has been found out and learned by some person or other, without the aid of an instructor. There is no species of learning, therefore, which even self-education may not overtake; for there is none which it has not actually overtaken. All discoverers (and the whole of human knowledge that has not been divinely revealed is the creation of discovery) have been self-taught at least in regard to that which they discovered. The person who first attempted the representation of sounds by writing must have taught himself his alphabet. This is the first consideration for all those who aspire, in the present day, to be their own instructors in any branch of science or literature. Furnished as society now is, in all its departments, with accommodations in aid of intellectual exertion, such as, in some respects, even the highest station and the greatest wealth in former times could not command, it may be safely asserted, that hardly any unassisted student can have any longer to encounter difficulties equal to those which have been a thousand times already triumphantly overcome.

1 "In point of fact, the first balloon sent up by the Montgolfiers at Annonay, near Lyons, the place where they carried on their paper manufactory, on the 5th of June, 1783, was raised simply by means of common air heated. It was not till the 27th of August following that the first balloon filled with bydrogen ascended from the Champs de Mars, Paris. The first attempt, nevertheless, of the Montgolfiers had been with hydrogen, but it proved a failure, as a similar attempt had Zone in the hands of Cavallo, at London, about the same time, in the year 1782. Long

before this the idea of rising into the sky by means of a ball formed of some light substance, and filled with inflammable air (or hydrogen), had occurred to Dr. Black of Edinburgh immediately on reading Mr Cavendish's announcement of the great levity of that gas, which was published in the Philosophical Transactions for 1766. Lord Brougham states (Lives of Men of Science, First Series, p. 337) that Black actually showed to a party of his friends the ascent of a bladder filled with inflammable air in that same year.

Above all, books, and especially elementary books, have, in our day, multiplied to an extent that puts them within the reach almost of the poorest student; and books, after all, are, at least to the more mature understanding, and in regard to such subjects as they are fitted to explain, the best teachers. He who can read, and is possessed of a good elementary treatise on the science he wishes to learn, hardly, in truth, needs a master. With only this assistance, and sometimes with hardly this, some of the greatest scholars and philosophers that ever appeared have formed themselves, as the following pages will show. And let him who, smitten by the love of knowledge, may yet conceive himself to be on any account unfortunately circumstanced for the business of mental cultivation, bethink him how often the eager student has made his way through a host of impediments, much more formidable in all probability than any by which he is surrounded. Want of leisure, want of instructors, want of books, poverty, ill-health, imprisonment, uncongenial or distracting occupations, the force of opposing example, the discouragement of friends or relations, the depressing consideration that the better part of life was already spent and gone—these have all, separately or in various combinations, exerted their influence either to check the pursuit of knowledge, or to prevent the very desire of it from springing up. But they opposed the force of the strong natural passion and upwardtending determination in vain. Here then is enough both of encouragement and of direction for all. To the illustrious vanquishers of fortune, whose triumphs we are about to record, we would point as pioneers and guides for all who, similarly circumstanced, may aspire to follow in the same honourable path. Their lives are lessons that cannot be read without profit. Nor are they lessons for the perusal of one class of society only. All, even those who are seemingly the most happily situated for the cultivation of their minds, may derive a stimulus from such anecdotes. No situation, in truth, is altogether without its unfavourable influences. If there be not poverty to crush the spirit, there may be too much wealth and too much ease, to relax and enervate it. He who is left to educate himself in everything, may have many difficulties to struggle with; but he who is permitted to educate himself in nothing is perhaps still more unfortunate. If one mind be in danger of starving for want of books, another may be surfeited by too many. If a laborious occupation leave to some but little time for study, there are temptations, it should be remembered, attendant upon rank and affluence, which are to the full as hard to escape from as any occupation. Or should there be any one who stands free, or comparatively free, from every kind of impediment to the cultivation of his intellectual faculties, he especially may be expected to feel a peculiar interest in the account of what the love of knowledge has achieved in circumstances so opposite to his own. It can hardly fail to stimulate his own exertions, and to

remind him that his acquisitions ought to be in some degree commensurate to his advantages. Finally, for all who love to read of bold and successful adventure, and to follow daring ambition in its career to greatness, it cannot but be interesting to contemplate the exploits of some of the most enterprising spirits of our race, the adventurers, namely, of the world of intellect, whose ambition, while it has soared as high, and performed feats as brilliant as any other, never excites in us an interest which it is dangerous to indulge, nor holds up to us an example which it would be criminal to follow.

CHAPTER II.

STRENGTH OF THE PASSION FOR KNOWLEDGE.-PYTHAGORAS;
ARCHIMEDES; LEIBNITZ; GALILEO; HEYNE.

THE ardour with which knowledge has frequently been pursued amidst all sorts of difficulties and discouragements, is the best evidence we can offer of the strength of the passion which has sprung up and lived in circumstances so unfavourable to its growth, and therefore of the exquisite pleasure which its gratification is found to bring with it. If the permanence of any pleasure, indeed, is to be looked upon as one of the elements of its preciousness, there are certainly none but those of virtue and religion that can be compared with the pleasures of intellectual exertion. Nor is successful study without its moments, too, of as keen and overpowering emotion, as any other species of human enjoyment is capable of yielding. We have seen how Newton was affected on approaching the completion of his sublime discovery; when the truth shone full upon him, and not a shade remained to create a doubt that it was indeed the truth which he had found and was looking upon. Every other discoverer, or inventor, or creator of any of the great works of literature or art, has had, doubtless, his moments of similar ecstasy. The ancient Greek philosopher PYTHAGORAS is said to have been the first who found out, or at least demonstrated, the great geometrical truth, that the square described on the hypothenuse, or side opposite to the right angle, of a right-angled triangle is exactly equal in area to the two squares described on the other two sides; and such was his joy, we are told, on the occasion, that he offered up a hecatomb, or sacrifice of a hundred oxen, to the gods, in testimony of his gratitude and exultation. When ARCHIMEDES, the Sicilian, the most renowned geometer of antiquity, achieved what we may call the completion of the method of ascertaining specific gravities, or the comparative weights of equal bulks of different substances, he is said to have rushed forth naked from the bath in which