Engines operated by means of hot air, called caloric engines, and engines operated by gas, or explosive substances, all act substantially upon the same general principles as steam-engines; the greatest distinction being between those engines wherein the generation of heat is by the combustion of fuel, and those wherein heat and expansion are produced by chemical action. With the exception of a limited number of caloric or air engines, steam machinery comprises nearly all expansive engines that are employed at this day for motive-power; and it may be safely assumed that a person who has mastered the general principles of steam-engines will find no trouble in analysing and understanding any machinery acting from expansion due to heat, whether air, gas, or explosive agents be employed.
To follow this matter further. It will be found in such machines as are directed mainly to augmenting force or increasing the amount of power that may be applied in any operation, such as sawing wood or stone, the effect produced when compared to hand labour is nearly as the difference in the amount of power applied; and the saving that such machines effect is generally in the same proportion. A machine that can expend ten horse-power in performing a certain kind of work, will save ten times as much as a machine directed to the same purpose expending but one horse-power; this of course applies to machines for the performance of the coarser kinds of work, and employed to supplant mere physical effort. In other machines of application, such as are directed mainly to guidance, or speed of action, such as sewing machines, dove-tailing machines, gear-cutting machines, and so on, there is no relation whatever between the increased  effect that may be produced and the amount of power expended.A book containing the pattern record should be kept, in which these catalogue numbers are set down, with a short description to identify the different parts to which the numbers belong, so that all the various details of any machine can at any time be referred to. Besides this description, there should be opposite the catalogue of pattern numbers, ruled spaces, in which to enter the weight of castings, the cost of the pattern, and also the amount of turned, planed, or bored surface on each piece when it is finished, or the time required in fitting, which is the same thing. In this book the assembled parts of each machine should be set down in a separate list, so that orders for castings can be made from the list without other references. This system is the best one known to the writer, and is in substance a plan now adopted in many of the best engineering establishments. A complete system in all things pertaining to drawings and classifications should be rigidly adhered to; any plan is better than none, and the schooling of the mind to be had in the observance of systematic rules is a matter not to be neglected. New plans for promoting system may at any time arise, but such plans cannot be at any  time understood and adopted except by those who have cultivated a taste for order and regularity.Civil engineering, in the meaning assumed for the term, has become almost a pure mathematical science. Constants are proved and established for nearly every computation; the strength and durability of materials, from long and repeated tests, has come to be well understood; and as in the case of machine tools, the uniformity of practice among civil engineers, and the perfection of their works, attest how far civil engineering has become a true science, and proves that the principles involved in the construction of permanent works are well understood.
In making designs it is best to employ no references except such as are carried in the memory. The more familiar a person is with machinery of any class, the more able he may be to prepare designs, but not by measuring and referring to other people's plans. Dimensions and arrangement from examples are, by such a course, unconsciously carried into a new drawing, even by the most skilled; besides, it is by no means a dignified matter to collect other people's plans, and by a little combination and modification produce new designs. It may be an easy plan to acquire a certain kind of proficiency, but will most certainly hinder an engineer from ever rising to the dignity of an original designer.
Designing machines must have reference to adaptation, endurance, and the expense of construction. Adaptation includes the performance of machinery, its commercial value, or what the machinery may earn in operating; endurance, the time that machines may operate without being repaired, and the constancy of their performance; expense, the investment represented in machinery.The following propositions in regard to tempering, comprehend the main points to be observed:CHAPTER XX. GENERALISATION OF SHOP PROCESSES.
Third. Machinery for applying power.
2. The necessity for, and consequent saving effected by, power-machinery for handling is mainly in vertical lifting, horizontal movement being easily performed by hand.
Draught, or the taper required to allow patterns to be drawn readily, is another of those indefinite conditions in pattern-making that must be constantly decided by judgment and experience. It is not uncommon to find rules for the draught of patterns laid down in books, but it would be difficult to find such rules applied. The draught may be one-sixteenth of an inch to each foot of depth, or it may be one inch to a foot of depth, or there may be no draught whatever. Any rule, considered aside from specified conditions, will only confuse a learner. The only plan to understand the proper amount of draught for patterns is to study the matter in connection with patterns and foundry operations.
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Second. Impact wheels, driven by the force of spouting water that expends its percussive force or momentum against the vanes tangental to the course of rotation, and at a right angle to the face of the vanes or floats.Second. Machinery for transmitting and distributing power.
A pressure wheel, like a steam-engine, must include running contact between water-tight surfaces, and like a rotary steam-engine, this contact is between surfaces which move at different rates of speed in the same joint, so that the wear is unequal, and increases as the speed or the distance from the axis. When it is considered that the most careful workmanship has never produced rotary engines that would surmount these difficulties in working steam, it can hardly be expected they can be overcome in using water, which is not only liable to be filled with grit and sediment, but lacks the peculiar lubricating properties  of steam. A rotary steam-engine is in effect the same as a pressure water-wheel, and the apprentice in studying one will fully understand the principles of the other.Steam and other machinery applied to the transport of material and travel, in navigation and by railways, comprises the greater share of what may be called engineering products; and when we consider that this vast interest of steam transport is less than a century old, and estimate its present and possible future influence on human affairs, we may realise the relation that mechanical science bears to modern civilisation.详情
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