INTRODUCTION. It is our intention, in these introductory pages, to make a few observations on the nature of scientific knowledge, which may be useful to the young reader in enabling him to understand more clearly the subjects contained in the volume, and in guarding him against the adoption of false theory, or the wasting of his time in inquiries which can terminate in no useful result. Such introductory observations are rendered the more necessary, as a correct knowledge of the subjects to which they relate, is the only sure foundation on which there can be raised a solid superstructure of science. It is a general opinion that scientific knowledge is entirely different from all other kinds of knowledge; or that it requires for its cultivation a constitution of mind only to be met with here and there in the great family of mankind; and what is said of the poet is also thought of the philosopher-that he is born, not made. All men are certainly not equally endowed with capacities for the acquisition of scientific knowledge, but there are few men indeed who are totally unprivileged. The man who would relinquish scientific pursuits merely because he had no hope of reaching the eminence of a Newton, a Watt, or a Davy, is no better than him, who, in despair of ever obtaining a share of wealth equal to that of the rich inheritor of the land, would cease to make any honest exertion to raise himself from a state of the most squalid wretchedness. We would not be understood by this to bring the acquisition of knowledge into invidious comparison with the acquisition of wealth--the one is in every case a godlike employment, but the other is often the concomitant of vice. The young mechanic should be made well aware that the knowledge of the man of science differs from the knowledge of ordinary men, nos so much in kind as in degree; and the knowLedge which guides the little boy in the erection of his summerhouse, constitutes a part of that knowledge which guides the best architect in the erection of the most splendid edifice. The boy raises his paper kite in the air, with no other end in view save his own amusement-he has learned to do so by seeing other boys do the same, and by trials he finds that the kite will fly oetter in a moderate wind than in a perfect calm, and that the weight at the tail may be too heavy or too light, and he regulates his actions accordingly so far he is a little philosopher. A man raises a kite knowing all that the boy knew, but he raises it with a view of determining the state of the atmosphere so far as electricity is concerned, for which purpose, instead of employing the hempen cord, which was sufficient for the purpose of the boy, ne employs a metallic wire, which he knows by experience will conduct the electricity from the clouds to the earth, and thus effects his design. In this respect the knowledge of the man is more extensive than that of the boy, but this additional knowledge has been obtained exactly in the same way as the knowledge of the boy, that is to say, by experience. Even the Indian, unlearned as he seems to be, is in some respects a philosopher. He sees daily that the paddle of his canoe is to appearance broken when he puts it into the water; but it is only to appearance, for by repeated trials, he finds that the paddle is as whole when in the water as when out of it. He knows also, by repeated trials, that the fish, while it shoots along through the clear flood, does not appear to be where it really is; for though the most unerring of marksmen, yet if he throws his dart directly at the point where the fish appears, he will certainly miss it. In vain will he try to strike the fish on the same principles as he strikes the bird flying in the air; but he finds, that when he directs his dart to a line which is nearer to him than that in which the fish seems to move, he will strike the fish. The Indian remembers the circumstance of his paddle, and other circumstances of a like kind, and concludes that, when bodies are viewed through water, they do not seem to be in the place in which they really are. When he knows and acts upon this principle, he is a man of science so far as this is concerned The man of science, indeed, as we commonly understand that appellation, knows much more than this: he knows that many other substances have a like effect in changing the apparent position of objects when seen through them nat one ru sa greater and another a less change, and by repeated trials he as.ertains the actual amount of their changes by measurement, and can subject them to the most rigid calculation; all of which knowledge is obtained in the same way as that of the Indian, but is more extensive. An examination of facts is the foundation of all true science; but science does not consist in a mere examination of facts. They must be compared with each other, and the general circumstance of their agreement carefully marked. When we have compared several facts together, and find that there is one general circumstance in which they agree, this one circumstance becomes, as it were, a chain by which they are all linked together. This general circumstance of agreement, when expressed in language, is what is called a law. For instance, it is a law that all bodies, when left to fall freely, will tend to the earth; and this law has been framed by us, because in all cases which we have examined this has been the case; and the term gravity, by which this law is designated, is nothing else than a name invented to express a circumstance in which we have found innumerable facts to agree. It was known for a very long time that water would not rise in a sucking pump to a height of more than thirty-two feet, and this was said to take place because nature abhorred a vacuum. The reason given was afterwards found to be false, yet the knowledge of the fact was exceedingly useful in the construction of pumps for lifting water. About the middle of the seventeenth century, Toricelli, the pupil of Galileo, made experiments on the subject, and found that fluids would rise in tubes or in sucking pumps higher in proportion as they were lighter; and collecting all the facts together, he concluded that the fluids were forced up by the pressure of the atmosphere, and thus laid down one of the most important laws of physical science. A collection of such laws which refers to some particular class of objects, when properly arranged, becomes what is called a theory. Thus we see that & theory, properly so called, is founded on an examination of particular facts, and of course cannot refer to any other but those facts which have been examined; or, if it is attempted so to do, it is no longer a theory, but an hypothesis or supposition. Hypotheses although they ought not to be relied upon, are nevertheless useful, as in our endeavours to discover whether they be true or false, we may at last ascertain the class of facts to which they belong, and thus arrive at the true theory. In the examination of facts, it is to be observed, that we must depend on the information derived through the medium of the five senses, that is, the senses of seeing-hearing-touching-tasting -and smelling;-for it is only by bodies affecting these organs that the properties of matter become known to us; and all that the mind does is to compare and classify the information thus derived. It is a common error to suppose that many of our greatest inventions and discoveries were made by accident. Many wonderful anecdotes are told in support of this assertion; but the very circumstance of their exciting our wonder is sufficient to show that they are out of the common course of our experience, and that, therefore, before they are received, they ought to undergo a careful examination. A multitude of facts might be adduced to prove that knowledge is more regularly progressive than is commonly imagined. Far be it from us to detract from the merits of those great men who have, from time to time, benefited mankind by their important discoveries; but from a survey of the history of science, we are led to the conviction, that whereever a new path has been struck out in the great field of truth, that path has been previously prepared by former inquirers. Had Kepler not discovered the three fundamental laws of the planetary motions, it is highly probable that the Principia of Newton never would have issued from the pen of that illustrious man; and had it not been for the brilliant discoveries of Dr. Black on the subject of heat, it is probable that Watt never would have made his improvements on the steam engine, that invaluable distributer of power. It is not unlikely, however, from the state of knowledge in the days of Newton, that, independent of the exertions of his mighty mind, the knowledge contained in the Principia would soon after have been given to the world by some one or more individuals—and the like may be said of the inventions of James Watt. The great lesson which we would wish the young mechanic to .earn from these observations is—that great discoveries are never made without preparation-that previous knowledge is necessary to turn what are called accidental occurrences to good account. And when he is told that the law of gravitation was suggested |