ONE:CHAPTER VI. A VISITOR.(1.) Why is the effect produced different on the top and bottom of a piece when struck by a hammer?(2.) Why does not a compound hammer create jar and concussion?(3.) What would be a mechanical difficulty in presenting the material to such hammers?(4.) Which is most important, speed or weight, in the effect produced on the under side of pieces, when struck by single acting hammers?
TWO:Bruce shrugged his shoulders indifferently.Inherent or cooling strains in castings is much more intricate than shrinkage: it is, in fact, one of the most uncertain and obscure matters that pattern-makers and moulders have to contend with. Inherent strains may weaken castings, or cause them to break while cooling, or sometimes even after they are finished; and in many kinds of works such strains must be carefully guarded against, both in the preparation of designs and the arrangement of patterns, especially for wheels and pulleys with spokes, and for struts or braces with both ends fixed. The main difficulty resulting from cooling strains, however, is that of castings being warped and sprung; this difficulty is continually present in the foundry and machine-shop, and there is perhaps no problem in the whole range of mechanical manipulation of which there exists more diversity of opinion and practice than of means to prevent the springing of castings. This being the case, an apprentice can hardly hope for much information here. There is no doubt of springing and strains in castings being the result of constant causes that might be fully understood if it were not for the ever-changing conditions which exist in casting, both as to the form of pieces, the temperature and quality of metal, mode of cooling, and so on.
ONE:This method of treating the subject of motive-engines will no doubt be presenting it in a new way, but it is merely beginning at an unusual place. A learner who commences with first principles, instead of pistons, valves, connections, and bearings, will find in the end that he has not only adopted the best course, but the shortest one to understand steam and other expansive engines.
TWO:But, towards the close of the sixteenth century, the time for amassing observations was past, no further progress being possible until the observations already recorded were interpreted aright. The just instinct of science perceived this; and for nearly a century after Cesalpino no addition of any magnitude was made to what Bacon called History, while mens conceptions of natural law were undergoing a radical transformation.544 To choose such a time for developing the Aristotelian philosophy was peculiarly unfortunate; for that philosophy had become, both on its good and on its bad side, an obstacle to progress, by encouraging studies which were not wanted, and by fostering a spirit of opposition to the Copernican astronomy.The signal went forward.
ONE:It is now proposed to review the standard tools of a fitting shop, noticing the general principles of their construction and especially of their operation; not by drawings nor descriptions to show what a lathe or a planing machine is, nor how some particular engineer has constructed such tools, but upon the plan explained in the introduction, presuming the reader to be familiar with the names and purposes of standard machine tools. If he has not learned this much, and does not understand the names and general objects of the several operations carried [121]on in a fitting shop, he should proceed to acquaint himself thus far before troubling himself with books of any kind.
TWO:I showed them the way to Eysden, and they had80 scarcely started when a cavalry patrol came racing on, the men tipsy and their seat rather unstable. Seeing the refugees, they aimed their rifles at them and roared "Hands up!" The poor creatures not only put up their hands, but fell on their knees, and muttered incoherent words. The women folded their hands, and stretched them out to the cavalry, as if praying for mercy. The soldiers looked at the scene for a moment, burst out in a harsh laughter, spurred on their horses, and raced on without a word. Two of them stopped near me. I gave them, however, no time for threats, but quickly showed them the old pass to Vis. As soon as they saw the German writing they said: "All right!" and went off.On passing to terrestrial physics, we find that Aristotle is, as usual, the dupe of superficial appearances, against which other thinkers were on their guard. Seeing that fire always moved up, he assumed that it did so by virtue of a natural tendency towards the circumference of the universe, as opposed to earth, which always moved towards the centre. The atomists erroneously held that all matter gravitated downwards through infinite space, but correctly explained the ascent of heated particles by the pressure of surrounding matter, in accordance, most probably, with the analogy of floating bodies.200 Chemistry as a science is, of course, an entirely modern creation, but the first approach to it was made by Democritus, while no ancient philosopher stood farther from its essential principles than Aristotle. He analyses bodies, not into their material elements, but into the sensuous qualities, hot and cold, wet and dry, between which he supposes the underlying substance to be perpetually oscillating; a theory which, if it were true, would make any fixed laws of nature impossible.
