THREE: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.In reasoning up from the world to its first cause, we were given to understand that the two were related to one another as contradictory opposites. The multiple must proceed from the simple, and existence from that which does not exist. But the analogies of material production now suggest a somewhat different view. What every power calls into existence is an image of itself, but the effect is never more than a weakened and imperfect copy of its original. Thus the universe appears as a series of diminishing energies descending in a graduated scale from the highest to the lowest. Here, again, bad science makes bad philosophy. Effects are never inferior to their causes, but always exactly equal, the effect being nothing else than the cause in another place or under another form. This would be obvious enough, did not superficial observation habitually confound the real320 cause with the sum of its concomitants. What we are accustomed to think of as a single cause is, in truth, a whole bundle of causes, which do not always converge to a single point, and each of which, taken singly, is, of course, inferior to the whole sum taken together. Thus when we say that the sun heats the earth, this is only a conventional way of speaking. What really does the work is a relatively infinitesimal part of the solar heat separately transmitted to us through space. Once neglect this truth, and there is no reason why effects should not exceed as well as fall short of their causes in any assignable proportion. Such an illusion is, in fact, produced when different energies converge to a point. Here it is the consequent and not the antecedent which is confounded with the sum of its concomitants, as when an explosion is said to be the effect of a spark.