in adding another to the large number of books which treat upon mechanics, and especially of that class devoted to what is called mechanical engineering, it will be proper to explain some of the reasons for preparing the present work; and as these explanations will constitute a part of the work itself, and be directed to a subject of some interest to a learner, they are included in the introduction.
first i will notice that among our many books upon mechanical subjects there are none that seem to be directed to the instruction of apprentice engineers; at least, there are none directed to that part of a mechanical education most difficult to acquire, a power of analysing and deducing conclusions from commonplace matters.
our text-books, such as are available for apprentices, consist mainly of mathematical formul? relating to forces, the properties of material, examples of practice, and so on, but do not deal with the operation of machines nor with constructive manipulation, leaving out that most important part of a mechanical education, which consists in special as distinguished from general knowledge.
the theorems, formul?, constants, tables, and rules, which are generally termed the principles of mechanics, are in a sense only symbols of principles; and it is possible, as many facts will prove, for a learner to master the theories and symbols of mechanical principles, and yet not be able to turn such knowledge [2] to practical account.
a principle in mechanics may be known, and even familiar to a learner, without being logically understood; it might even be said that both theory and practice may be learned without the power to connect and apply the two things. a person may, for example, understand the geometry of tooth gearing and how to lay out teeth of the proper form for various kinds of wheels, how to proportion and arrange the spokes, rims, hubs, and so on; he may also understand the practical application of wheels as a means of varying or transmitting motion, but between this knowledge and a complete wheel lies a long train of intricate processes, such as pattern-making, moulding, casting, boring, and fitting. farther on comes other conditions connected with the operation of wheels, such as adaptation, wear, noise, accidental strains, with many other things equally as important, as epicycloidal curves or other geometrical problems relating to wheels.
text-books, such as relate to construction, consist generally of examples, drawings, and explanations of machines, gearing, tools, and so on; such examples are of use to a learner, no doubt, but in most cases he can examine the machines themselves, and on entering a shop is brought at once in contact not only with the machines but also with their operation. examples and drawings relate to how machines are constructed, but when a learner comes to the actual operation of machines, a new and more interesting problem is reached in the reasons why they are so constructed.
the difference between how machinery is constructed and why it is so constructed, is a wide one. this difference the reader should keep in mind, because it is to the second query that the present work will be mainly addressed. there will be an attempt—an imperfect one, no doubt, in some cases—to deduce from practice the causes which have led to certain forms of machines, and to the ordinary processes of workshop manipulation. in the mind of a learner, whether apprentice or student, the strongest tendency is to investigate why certain proportions and arrangement are right and others wrong—why the operations of a workshop are conducted in one manner instead of another? this is the natural habit of thought, and the natural course of inquiry and investigation is deductive.
nothing can be more unreasonable than to expect an apprentice engineer to begin by an inductive course in learning and reasoning [3] about mechanics. even if the mind were capable of such a course, which can not be assumed in so intricate and extensive a subject as mechanics, there would be a want of interest and an absence of apparent purpose which would hinder or prevent progress. any rational view of the matter, together with as many facts as can be cited, will all point to the conclusion that apprentices must learn deductively, and that some practice should accompany or precede theoretical studies. how dull and objectless it seems to a young man when he toils through "the sum of the squares of the base and perpendicular of a right-angle triangle," without knowing a purpose to which this problem is to be applied; he generally wonders why such puzzling theorems were ever invented, and what they can have to do with the practical affairs of life. but if the same learner were to happen upon a builder squaring a foundation by means of the rule "six, eight, and ten," and should in this operation detect the application of that tiresome problem of "the sum of the squares," he would at once awake to a new interest in the matter; what was before tedious and without object, would now appear useful and interesting. the subject would become fascinating, and the learner would go on with a new zeal to trace out the connection between practice and other problems of the kind. nothing inspires a learner so much as contact with practice; the natural tendency, as before said, is to proceed deductively.
a few years ago, or even at the present time, many school-books in use which treat of mechanics in connection with natural philosophy are so arranged as to hinder a learner from grasping a true conception of force, power, and motion; these elements were confounded with various agents of transmission, such as wheels, wedges, levers, screws, and so on. a learner was taught to call these things "mechanical powers," whatever that may mean, and to compute their power as mechanical elements. in this manner was fixed in the mind, as many can bear witness, an erroneous conception of the relations between power and the means for its transmission; the two things were confounded together, so that years, and often a lifetime, has not served to get rid of the idea of power and mechanism being the same. to such teaching can be traced nearly all the crude ideas of mechanics so often met with among those well informed in other matters. in the great change from empirical rules to proved constants, from special and experimental knowledge to the application of science [4] in the mechanic arts, we may, however, go too far. the incentives to substitute general for special knowledge are so many, that it may lead us to forget or underrate that part which cannot come within general rules.
the labour, dirt, and self-denial inseparable from the acquirement of special knowledge in the mechanic arts are strong reasons for augmenting the importance and completeness of theoretical knowledge, and while it should be, as it is, the constant object to bring everything, even manipulative processes, so far as possible, within general rules, it must not be forgotten that there is a limit in this direction.
in england and america the evils which arise from a false or over estimate of mere theoretical knowledge have thus far been avoided. our workshops are yet, and must long remain, our technological schools. the money value of bare theoretical training is so fast declining that we may be said to have passed the point of reaction, and that the importance of sound practical knowledge is beginning to be more felt than it was some years ago. it is only in those countries where actual manufactures and other practical tests are wanting, that any serious mistake can be made as to what should constitute an education in mechanics. our workshops, if other means fail, will fix such a standard; and it is encouraging to find here and there among the outcry for technical training, a note of warning as to the means to be employed.
during the meeting of the british association in belfast (1874), the committee appointed to investigate the means of teaching physical science, reported that "the most serious obstacle discovered was an absence from the minds of the pupils of a firm and clear grasp of the concrete facts forming a base of the reasoning processes they are called upon to study; and that the use of text-books should be made subordinate to an attendance upon lectures and demonstrations."
here, in reference to teaching science, and by an authority which should command our highest confidence, we have a clear exposition of the conditions which surround mechanical training, with, however, this difference, that in the latter "demonstration" has its greatest importance.
professor john sweet of cornell university, in america, while delivering an address to the mechanical engineering classes, during the same year, made use of the following words: "it is [5] not what you 'know' that you will be paid for; it is what you can 'perform,' that must measure the value of what you learn here." these few words contain a truth which deserves to be earnestly considered by every student engineer or apprentice; as a maxim it will come forth and apply to nearly everything in subsequent practice.
i now come to speak directly of the present work and its objects. it may be claimed that a book can go no further in treating of mechanical manipulation than principles or rules will reach, and that books must of necessity be confined to what may be called generalities. this is in a sense true, and it is, indeed, a most difficult matter to treat of machine operations and shop processes; but the reason is that machine operations and shop processes have not been reduced to principles or treated in the same way as strains, proportions, the properties of material, and so on. i do not claim that manipulative processes can be so generalised—this would be impossible; yet much can be done, and many things regarded as matters of special knowledge can be presented in a way to come within principles, and thus rendered capable of logical investigation.
writers on mechanical subjects, as a rule, have only theoretical knowledge, and consequently seldom deal with workshop processes. practical engineers who have passed through a successful experience and gained that knowledge which is most difficult for apprentices to acquire, have generally neither inclination nor incentives to write books. the changes in manipulation are so frequent, and the operations so diversified, that practical men have a dread of the criticisms which such changes and the differences of opinion may bring forth; to this may be added, that to become a practical mechanical engineer consumes too great a share of one's life to leave time for other qualifications required in preparing books. for these reasons "manipulation" has been neglected, and for the same reasons must be imperfectly treated here. the purpose is not so much to instruct in shop processes as to point out how they can be best learned, the reader for the most part exercising his own judgment and reasoning powers. it will be attempted to point out how each simple operation is governed by some general principle, and how from such operations, by tracing out the principle which lies at the bottom, it is possible to deduce logical conclusions as to what is right or wrong, expedient or inexpedient. in this way, it is thought, can [6] be established a closer connection between theory and practice, and a learner be brought to realise that he has only his reasoning powers to rely on; that formul?, rules, tables, and even books, are only aids to this reasoning power, which alone can master and combine the symbol and the substance.
no computations, drawings, or demonstrations of any kind will be employed to relieve the mind of the reader from the care of remembering and a dependence on his own exertions. drawings, constants, formul?, tables, rules, with all that pertains to computation in mechanics, are already furnished in many excellent books, which leave nothing to be added, and such books can be studied at the same time with what is presented here.
the book has been prepared with a full knowledge of the fact, that what an apprentice may learn, as well as the time that is consumed in learning, are both measured by the personal interest felt in the subject studied, and that such a personal interest on the part of an apprentice is essential to permanent success as an engineer. a general dryness and want of interest must in this, as in all cases, be a characteristic of any writing devoted to mechanical subjects: some of the sections will be open to this charge, no doubt, especially in the first part of the book; but it is trusted that the good sense of the reader will prevent him from passing hurriedly over the first part, to see what is said, at the end, of casting, forging, and fitting, and will cause him to read it as it comes, which will in the end be best for the reader, and certainly but fair to the writer.