The successful application of the pneumatic system at the Mont Cenis Tunnel in Italy, and at the Hoosac Tunnel in America, has demonstrated the value of the system where the air not only served to transmit power to operate the machinery but to ventilate the mines at the same time. Air brakes for railway trains are another example illustrating the advantages of pneumatic transmission; the force being multiplied at the points where it is applied, so that the connecting pipes are reduced to a small size, the velocity of the air making up for a great force that formerly had to be communicated through rods, chains, or shafts. The principal object attained by the use of air to operate railway brakes is, however, to maintain a connection throughout a train by means of flexible pipes that accommodate themselves to the varying distance between the carriages. Presuming that the flow of air in pipes is not materially impeded by friction or angles, and that there will be no difficulty in maintaining lubrication for pistons or other inaccessible parts of machinery when driven by air, there seems to be many reasons in favour of its use as a means of distributing power in manufacturing districts. The diminished cost of motive power when  it is generated on a large scale, and the expense and danger of maintaining an independent steam power for each separate establishment where power is employed, especially in cities, are strong reasons in favour of generating and distributing power by compressed air, through pipes, as gas and water are now supplied.(1.) What kind of strains are shafts subjected to?—(2.) What determines the strength of shafts in resisting transverse strain?—(3.) Why are shafts often more convenient than belts for transmitting power?—(4.) What is the difference between the strains to which shafts and belts are subjected?—(5.) What is gained by constructing a line shaft of sections diminishing in size from the first mover?—(6.) What is gained by constructing line shafts of uniform diameter?There is, perhaps, no one who has achieved a successful experience as an engineer but will acknowledge the advantages derived from early efforts to generate original designs, and none who will not admit that if their first efforts had been more carefully directed, the advantages gained would have been greater.It may also be remarked that the special knowledge involved in applied mechanics is mainly to be gathered and retained by personal observation and memory, and that this part is the greater one; all the formul? relating to machine construction may be learned in a shorter time than is required to master and understand the operations which may be performed on an engine  lathe. Hence first lessons, learned when the mind is interested and active, should as far as possible include whatever is special; in short, no opportunity of learning special manipulation should be lost. If a wheel pattern come under notice, examine the manure in which it is framed together, the amount of draught, and how it is moulded, as well as to determine whether the teeth have true cycloidal curves. 飞艇助赢计划app (1.) If milling tools operate faster than planing or turning tools, why are they not more employed?—(2.) How may the effect produced by cutting tools generally be computed?—(3.) To what class of work are milling machines especially suited?—(4.) Why do milling processes produce more accurate dimensions than are attainable by turning or planing?—(5.) Why can some branches of manufacture be said to depend on milling processes?At the risk of laying down a proposition not warranted by science, I will mention, in connection with this matter of crystallisation, that metal when disposed in the form of a ring, for some strange reason seems to evade the influences which produce crystalline change. A hand-hammer, for example, may be worn away and remain fibrous; the links of chains and the tires of waggon wheels do not become crystallised; even the tires on locomotive wheels seem to withstand this influence, although the conditions of their use are such as to promote crystallisation.An excellent plan to retain what is learned, is to make notes. There is nothing will assist the memory more in learning mechanics than to write down facts as they are learned, even if such memoranda are never referred to after they are made. Motive machinery.First.—The power is connected to the hammer by means of the least possible mechanism, consisting only of a cylinder, a piston, and slide valve, induction pipe and throttle valve; these few details taking the place of a steam-engine, shafts, belts, cranks, springs, pulleys, gearing, in short, all such details as are required between the hammer-head and the steam-boiler in the case of trip-hammers or crank-hammers.The expense of forming pattern-moulds may be considered as divided between the foundry and pattern-shop. What a pattern-maker saves a moulder may lose, and what a pattern-maker spends a moulder may save; in other words, there is a point beyond which saving expense in patterns is balanced by extra labour and waste in moulding—a fact that is not generally realised because of inaccurate records of both pattern and foundry work. What is lost or saved by judicious or careless management in the matter of patterns and moulding can only be known to those who are well skilled in both moulding and pattern-making. A moulder may cut all the fillets in a mould with a trowel; he may stop off, fill  up, and print in, to save pattern-work, but it is only expedient to do so when it costs very much less than to prepare proper patterns, because patching and cutting in moulds seldom improves them.The functions required in machinery for handling material in a machine shop correspond very nearly to those of the human hands. Nature in this, as in all other things, where a comparison is possible, has exceeded man in adaptation; in fact, we cannot conceive of anything more perfect than the human hands for handling material—a duty that forms a great share of all that we term labour.Workshop processes which are capable of being systematised are the most easy to learn. When a process is reduced to a system it is no longer a subject of special knowledge, but comes within general rules and principles, which enable a learner to use his reasoning powers to a greater extent in mastering it. This importance of shop processes in machine construction is generally realised by proprietors, but not thoroughly understood in all of its bearings; an apprentice may notice the continual effort that is made to augment the production of engineering-works, which is the same thing as shortening the processes.To estimate how much is yet to be learned in mechanical engineering, we have only to apply the same test, and when we contrast the great variance between the designs of machines and the diversity of their operation, even when applied to similar purposes, their imperfection is at once apparent. It must, however, be considered that if the rules of construction were uniform, and the principles of machine operation as well understood as the strength and arrangement of material in permanent structures, still there would remain the difficulty of adaptation to new  processes, which are continually being developed. It is not the intention to discuss patent law, nor even to estimate what benefits have in the past, or may in the future, be gained to technical industry by the patent system, but to impress engineering apprentices with a better and more dignified appreciation of their calling than to confound it with chance invention, and thereby destroy that confidence in positive results which has in the past characterised mechanical engineering; also to caution learners against the loss of time and effort too often expended  in searching after inventions.Standard tools for turning, drilling, planing, boring, and so on, have been changed but little during twenty years past, and are likely to remain quite the same in future. A lathe or a planing-machine made by a first-class establishment twenty years ago has, in many cases, the same capacity, and is worth nearly as much in value at the present time as machine tools of modern construction—a test that more than any other determines their comparative efficiency and the true value of the improvements that have been made. The plans of the framing for machine tools have been altered, and many improvements in details have been added; yet, upon the whole, it is safe to assume, as before said, that standard tools for metal-cutting have reached a state of improvement that precludes any radical changes in future, so long as the operations in metal-cutting remain the same.An apprentice in entering the workshop should avoid everything tending to an appearance of fastidiousness, either of manner or dress; nothing is more repulsive to workmen, and it may be added, nothing is more out of place in a machine shop than to divide one's time between the work and an attempt to keep clean. An effort to keep as neat as the nature of the work will admit is at all times right, but to dress in clothing not appropriate, or to allow a fear of grease to interfere with the performance of work, is sure to provoke derision.The use of hand tools should be learned by employing them on every possible occasion. A great many of the modern improvements in engine lathes are only to evade hand tool work, and in many cases effect no saving except in skill. A latheman who is skilful with hand tools will, on many kinds of light work, perform more and do it better on a hand lathe than an engine lathe; there is always more or less that can be performed to advantage with hand tools even on the most elaborate engine lathes. 飞艇助赢计划app 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.Proceeding from these premises, the first thing should be to examine the action of existing valve gear, to determine where this want of the dead-stroke function can best be supplied, and to gain the aid of such suggestions as existing mechanism may offer, also to see how far the appliances in use may become a part of any new arrangement.It will scarcely be expected that any part of the present work, intended mainly for apprentice engineers, should relate to designing machines, yet there is no reason why the subject should not to some extent be treated of; it is one sure to engage more or less attention from learners, and the study of designing machines, if properly directed, cannot fail to be of advantage. Besides the machine tools named, there are special machines to be found in most works, machines directed to the performance of certain work; by a particular adaptation such machines are rendered more effective, but they are by such adaptation unfitted for general purposes.Dimension lines should be in blue, but may be in red. Where to put them is a great point in draughting. To know where dimensions are required involves a knowledge of fitting and pattern-making, and cannot well be explained; it must be learned in practice. The lines should be fine and clear, leaving a space in their centre for figures when there is room. The distribution of centre lines and dimensions over a drawing must be carefully studied, for the double purpose of giving it a good appearance and to avoid confusion. Figures should be made like printed numerals; they are much better understood by the workman, look more artistic, and when once learned require but little if any more time than written figures. If the scale employed is feet and inches, dimensions to three feet should be in inches, and above this in feet and inches; this corresponds to shop custom, and is more comprehensive to the workman, however wrong it may be according to other standards. 7. By condensing the steam before it leaves the engine, so that the steam is returned to the air in the form of water, and of the same volume as when it entered the boiler, there is a gain  effected by avoiding atmospheric pressure, varying according to the perfection of the arrangements employed.Designing, or generating the plans of machinery, may be considered the leading element in engineering manufactures or machine construction, that one to which all others are subordinate,  both in order and importance, and is that branch to which engineering knowledge is especially directed. Designing should consist, first, in assuming certain results, and, secondly, in conceiving of mechanical agents to produce these results. It comprehends the geometry of movements, the disposition and arrangement of material, the endurance of wearing surfaces, adjustments, symmetry; in short, all the conditions of machine operation and machine construction. This subject will be again treated of at more length in another section.Invention, as applied to mechanical improvements, should not mean chance discovery. Such a meaning is often, if not generally, attached to the term invention, yet it must be seen that results attained by a systematic course of reasoning or experimenting can have nothing to do with chance or even discovery. Such results partake more of the nature of demonstrations, a name peculiarly suitable for such inventions as are the result of methodical purpose. 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.These various lathes, although of a widely varied construction and adapted to uses more or less dissimilar, are still the engine lathe either with some of its functions omitted to simplify and adapt it to some special work, or with some of the operative parts compounded to attain greater capacity.Forging relates to shaping metal by compression or blows when it is in a heated and softened condition; as a process, it is an intermediate one between casting and what may be called the cold processes. Forging also relates to welding or joining  pieces together by sudden heating that melts the surface only, and then by forcing the pieces together while in this softened or semi-fused state. Forging includes, in ordinary practice, the preparation of cutting tools, and tempering them to various degrees of hardness as the nature of the work for which they are intended may require; also the construction of furnaces for heating the material, and mechanical devices for handling it when hot, with the various operations for shaping, which, as in the case of casting, can only be fully understood by experience and observation.2. Drawings in true elevation or in section are based upon flat planes, and give dimensions parallel to the planes in which the views are taken. 飞艇助赢计划app Shaping machines are generally provided with adjustable vices, devices for planing circular forms, and other details which cannot be so conveniently employed with planing machines. Another feature of shaping machines is a positive range of the cutting stroke produced by crank motion, which permits tools to be stopped with precision at any point; this admits of planing  slots, keyways, and such work as cannot well be performed upon common planing machines.Handling and moving material is the principal matter to be considered in the arrangement of engineering works. The constructive manipulation can be watched, estimated, and faults detected by comparison, but handling, like the designs for machinery, is a more obscure matter, and may be greatly at fault without its defects being apparent to any but those who are highly skilled, and have had their attention especially directed  to the matter.One of the problems connected with the handling of material is to determine where hand-power should stop and motive-power begin—what conditions will justify the erection of cranes, hoists, or tramways, and what conditions will not. Frequent mistakes are made in the application of power when it is not required, especially for handling material; the too common tendency of the present day being to apply power to every purpose where it is possible, without estimating the actual saving that, may be effected. A common impression is that motive power, wherever applied to supplant hand labour in handling material, produces a gain; but in many cases the  fallacy of this will be apparent, when all the conditions are taken into account.Between a running motion of the dies, or a running motion of the blanks, there are the following points which may be noticed. It is best, when an apprentice thinks of entering an engineering establishment, to inquire of its character from disinterested persons who are qualified to judge of the facilities it affords. As a rule, every machine-shop proprietor imagines his own establishment to combine all the elements of an engineering business—and the fewer the facilities for learners, usually the more extravagant this estimate; so that opinions in the matter,  to be relied upon, should come from disinterested sources.In respect to the difference between expanding and solid dies it consists mainly in the time required to run back, and the injury to dies which this operation occasions. Uniformity of  size is within certain limits insured by solid dies, but they are more liable to derangement and less easy to repair than expanding or independent dies.This article has been introduced, not only to give a true understanding of the effect and value of machine combination, but to caution against a common error of confounding machine combination with invention.