REESE LIBRARY OF THI: UNIVERSITY OF CALIFORNIA. Received. L//U24&/ . J^ Accessions No. ^4 that the steam is returned to the air in the form of water, and C 34 WORKSHOP MANIPULATION. 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. 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 under- standing any machinery acting from expansion due to heat, whether air, gas, or explosive agents be employed. 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 prin- ciples, 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 ,(1.) What is principal among the details of steam machinery ? (2.) What has been the most important improvement recently made in steam machinery ? (3.) What has been the result of expansive engines generally stated ? (4.) Why has water proved the most successful among various expansive substances employed to develop power ? (5.) Why does a condensing engine develop more power than a non-con- densing one ? (6.) How far back from its development into power can heat be traced as an element in nature 1 (7.) Has the property of com- bustion a common source in all substances ? WATER-POWER. 35 CHAPTER VIII. WATER-POWER. WATER-WHEELS, next to steam-engines, are the most common motive agents. For centuries water-wheels remained without much improvement or change down to the period of turbine wheels, when it was discovered that instead of being a very simple matter, the science of hydraulics and water-wheels involved some very intricate conditions, giving rise to many problems of scientific interest, that in the end have produced the class known as turbine wheels. A modern turbine water-wheel, one of the best construction, operating under favourable conditions, gives a percentage of the power of the water which, after deducting the friction of the wheel, almost reaches the theoretical coefficient or equals the gravity of the water; it may therefore be assumed that there will in the future be but little improvement made in such water-wheels except in the way of simplifying and cheapening their construction. There is, in fact, no other class of machines which seem to have reached the same state of improvement as water-wheels, nor any other class of machinery that is con- structed with as much uniformity of design and arrangement, in different countries, and by different makers. Water-wheels, or water-power, as a mechanical subject, is apparently quite disconnected with shop manipulation, but will serve as an example for conveying general ideas of force and motion, and, on these grounds, will warrant a more extended notice than the seeming connection with the general subject calls for. In the remarks upon steam-engines it was explained that power is derived from heat, and that the water and the engine were both to be regarded as agents through which power was applied, and further, that power is always a product of heat. There is, perhaps, no problem in the whole range of mechanics more interesting than to trace the application of this principle in machinery ; one that is not only interesting but instructive, and may suggest to the mind of an apprentice a course of 36 WORKSHOP MANIPULATION. investigation that will apply to many other matters connected with power and mechanics. Power derived from water by means of wheels is due to the gravity of the water in descending from a higher to a lower level ; but the question arises, What has heat to do with this 1 If heat is the source of power, and power a product of heat, there must be a connection somewhere between heat and the descent of the water. Water, in descending from one level to another, can give out no more power than was consumed in raising it to the higher level, and this power employed to raise the water is found to be heat. Water is evaporated by heat of the sun, expanded until it is lighter than the atmosphere, rises through the air, and by condensation falls in the form of rain over the earth's surface; then drains into the ocean through streams and rivers, to again resume its round by another course of evaporation, giving out in its descent power that we turn to useful account by means of water- wheels. This principle of evaporation is continually going on ; the fall of rain is likewise quite constant, so that streams are maintained within a sufficient regularity to be available for operating machinery. The analogy between steam-power and water-power is there- fore quite complete. Water is in both cases the medium through which power is obtained; evaporation is also the leading principle in both, the main difference being that in the case of steam-power the force employed is directly from the expansion of water by heat, and in water-power the force is an indirect result of expansion of water by heat. Every one remembers the classification of water-wheels met with in the older school-books on natural philosophy, where we are informed that there are three kinds of wheels, as there were "three kinds of levers" namely, overshot, undershot, and breast wheels with a brief notice of Barker's mill, which ran apparently without any sufficient cause for doing so. Without finding fault with the plan of describing water-power commonly adopted in elementary books, farther than to say that some explana- tion of the principles by which power is derived from the water would have been more useful, I will venture upon a different classification of water-wheels, more in accord with modern practice, but without reference to the special mechanism of the different wheels, except when unavoidable. Water-wheels can be divided into four general types. WATER-POWER. 37 First. Gravity wheels, acting directly from the weight of the water which is loaded upon a wheel revolving in a vertical plane, the weight resting upon the descending side until the water has reached the lowest point, where it is discharged. 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. Third. Reaction wheels, that are "enclosed," as it is termed, and filled with water, which is allowed to escape under pressure through tangental orifices, the .propelling force being derived from the unbalanced pressure within the wheel, or from the re- action due to the weight and force of the water thrown off from the periphery. Fourth. Pressure wheels, acting in every respect upon the principle of a rotary steam-engine, except in the differences that arise from operating with an elastic and a non-elastic fluid ; thk pressure of the water resting continually against the vanes and "abutment," without means of escape except by the rotation of the wheel. To this classification may be added combinationUwhe^fej/'' acting partly by the gravity and partly by the percussiaktfhrce "& f of the water, by impact combined with reaction, or by nW!$5^ and maintained pressure. Gravity, or "overshot" wheels, as they are called, for some reasons will seem to be the most effective, and capable of utilis- ing the whole effect due to the gravity of the water ; but in practice this is not the case, and it is only under peculiar con- ditions that wheels of this class are preferable to turbine wheels, and in no case will they give out a greater per cent, of power than turbine wheels of the best class. The reasons for this will be apparent by examining the conditions of their operation. A gravity wheel must have a diameter equal to the fall of water, or, to use the technical name, the height of the head. The speed at the periphery of the wheel cannot well exceed sixteen feet per second without losing a part of the effect by the wheel anticipating or overrunning the water. This, from the large diameter of the wheels, produces a very slow axial speed, and a train of multiplying gearing becomes necessary in order to reach the speed required in most operations where power is 33 WORKSHOP MANIPULATION. applied. This train of gearing, besides being liable to wear and accident, and costing usually a large amount as an investment, consumes a considerable part of the power by frictional resist- ance, especially when such gearing consists of tooth wheels. Gravity wheels, from their large size and their necessarily ex- posed situation, are subject to be frozen up in cold climates ; and as the parts are liable to be first wet and then dry, or warm and cold by exposure to the air and the water alternately, the tendency to corrosion if constructed of iron, or to decay if of wood, is much greater than in submerged wheels. Gravity wheels, to realise the highest measure of effect from the water, require a diameter so great that they must drag in the water at the bottom or delivering side, and are for this reason especially affected by back-water, to which all wheels are more or less liable from the reflux of tides or by freshets. These disadvan- tages are among the most notable pertaining to gravity wheels, and have, with other reasons such as the inconvenience of con- struction, greater cost, and so on driven such wheels out of use by the force of circumstances, rather than by actual tests or theoretical deductions. Impact wheels, or those driven by the percussive force of water, including the class termed turbine water-wheels, are at this time generally employed for heads of all heights. The general theory of their action may be explained in the following propositions : 1. The spouting force of water is theoretically equal to its gravity. 2. The percussive force of spouting water can be fully utilised if its motion is altogether arrested by the vanes of a wheel. 3. The force of the water is greatest by its striking against planes at right angles to its course. 4. Any force resulting from water rebounding from the vanes parallel to their face, or at any angle not reverse to the motion of the wheel, is lost. 5. This rebounding action becomes less as the columns of water projected upon the wheel are increased in number and diminished in size. 6. To meet the conditions of rotation in the wheel, and to facilitate the escape of the water without dragging, after it has expended its force upon the vanes, the reversed curves of the turbine is the best-known arrangement. WATER-POWER. 39 It is, of course, very difficult to deal with so complex a subject as the present one with words alone, and the reader is recom- mended to examine drawings, or, what is better, water-wheels themselves, keeping the above propositions in view. Modern turbine wheels have been the subject of the most careful investigation by able engineers, and there is no lack of mathematical data to be referred to and studied after the general principles are understood. The subject, as said, is one of great complicity if followed to detail, and perhaps less useful to a mechanical engineer who does not intend to confine his practice to water-wheels, than other subjects that may be studied with greater advantage. The subject of water-wheels may, indeed, be called an exhausted one that can promise but little return for labour spent upon it with a view to improvements, at least. The efforts of the ablest hydraulic engineers have not added much to the percentage of useful effect realised by turbine wheels during many years past. Keaction wheels are employed to a limited extent only, and will soon, no doubt, be extinct as a class of water-wheels. In speaking of reaction wheels, I will select what is called Barker's mill for an example, because of the familiarity with which it is known, although its construction is greatly at variance with modern reaction wheels. There is a problem as to the principle of action in a Barker wheel, which although it may be very clear in a scientific sense, remains a puzzle to the minds of many who are well versed in mechanics, some contending that the power is directly from pressure, others that it is from the dynamic effect due to reaction. It is one of the problems so difficult to determine by ordinary standards, that it serves as a matter of endless debate between those who hold different views ; and considering the advantage usually derived from such controversies, perhaps the best manner of disposing of the problem here is to state the two sides as clearly as possible, and leave the reader to determine for himself which he thinks right. Presuming the vertical shaft and the horizontal arms of a Barker wheel to be filled with water under a head of sixteen feet, there would be a pressure of about seven pounds upon each superficial inch of surface within the cross arm, exerting an equal force in every direction. By opening an orifice at the sides of these arms equal to one inch of area, the pressure would at that 40 WORKSHOP MANIPULATION. point be relieved by the escape of the water, and the internal pressure be unbalanced to that extent. In other words, opposite this orifice, and on the other side of the arm, there would be a force of seven pounds, which being unbalanced, acts as a pro- pelling power to drive the wheel. This is one theory of the principle upon which the Barker wheel operates, which has been laid down in Vogdes' " Mensura- tion," and perhaps elsewhere. The other theory alluded to is that, direct action and reaction being equal, ponderable matter discharged tangentally from the periphery of a wheel must create a reactive force equal to the direct force with which the weight is thrown off. To state it more plainly, the spouting water that issues from the arm of a Barker wheel must react in the opposite course in proportion to its weight. The two propositions may be consistent with each other er even identical, but there still remains an apparent difference. The latter seems a plausible theory, and perhaps a correct one ; but there are two facts in connection with the operation of reaction water-wheels which seem to controvert the latter and favour the first theory, namely, that reaction wheels in actual practice seldom utilise more than forty per cent, of useful effect from the water, and that their speed may exceed the initial velocity of the water. With this the subject is left as one for argument or investigation on the part of the reader. Pressure wheels, like gravity wheels, should, from theoretical inference, be expected to give a high per cent, of power. The water resting with the whole of its weight against the vanes or abutments, and without chance of escape except by turning the wheel, seems to meet the conditions of realising the whole effect due to the gravity of the water, and such wheels would no doubt be economical if they had not to contend with certain mechanical difficulties that render them impracticable in most cases. 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 diffi- culties in working steam, it can hardly be expected they can be overcome in using water, which is not only liable to be filled WIND-POWER. 41 with grit and sediment, but lacks the peculiar lubricating pro- perties 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. (1.) What analogy may be found between steam and water power 1 ? (2.) What is the derivation of the name turbine ? (3.) To what class of water-wheels is this name applicable 1 (4.) How may water-wheels be classified? (5.) Upon what principle does a reaction water-wheel operate ? (6.) Can ponderable weight and pressure be independently considered in the case? (7.) Why cannot radial running joints be maintained in machines ? (8.) Describe the mechanism in common use for sustaining the weight of turbine wheels, and the thrust of propeller shafts. CHAPTER IX, WIND-POWER. WIND-POWER, aside from the objections of uncertainty and irreg- ularity, is the cheapest kind of motive-power. Steam machinery, besides costing a large sum as an investment, is continually deteriorating in value, consumes fuel, and requires continual skilled attention. Water-power also requires a large investment, greater in many cases than steam-power, and in many places the plant is in danger of destruction by freshets. Wind-power is less expensive in every way, but is unreliable for constancy except in certain localities, and these, as it happens, are for the most part distant from other elements of manufacturing industry. The operation of wind- wheels is so simple and so generally under- stood that no reference to mechanism need be made here. The force of the wind, moving in right lines, is easily applied to producing rotary motion, the difference from water-power being mainly in the comparative weakness of wind currents and the greater area required in the vanes upon which the wind acts. Turbine wind-wheels have been constructed on very much the same plan as turbine water-wheels. In speaking of wind-power, the propositions about heat must not be forgotten. It has been ex- plained how heat is almost directly utilised by the steam-engine, 42 WORKSHOP MANIPULATION. and how the effect of heat is utilised by water-wheels in a less direct manner, and the same connection will be found between heat and wind-wheels or wind-power. Currents of air are due to changes of temperature, and the connection between the heat that produces such air currents and their application as power is no more intricate than in the case of water-power. (1.) What is the difference in general between wind and water wheels 1 (2.) Can the course of wind, like that of water, be diverted and applied at pleasure ? (3.) On what principle does wind act against the vanes of a wheel ? (4.) How may an analogy between wind-power and heat be traced 1 CHAPTER X. MACHINERY FOR TRANSMITTING AND DISTRIBUTING POWER. To construe the term ''transmission of power" in its full sense, it will, when applied to machinery, include nearly all that has motion ; for with the exception of the last movers, or where power passes off and is expended upon work that is performed, all machinery of whatever kind may be called machinery of transmission. Custom has, however, confined the use of the term to such devices as are employed to convey power from one place to another, without including organised machines through which power is directly applied to the performance of work. Power is transmitted by means of shafts, belts, friction wheels, gearing, and in some cases by water or air, as various conditions of the work to be performed may require. Sometimes such machinery is employed as the conditions do not require, because there is, perhaps, nothing of equal importance connected with mechanical engineering of which there exists a greater diversity of opinion, or in which there is a greater diversity of practice, than in devices for transmitting power. I do not refer to questions of mechanical construction, although the remark might be true if applied in this sense, but to the kind of devices that may be best employed in certain cases. TRANSMITTING MACHINERY. 43 It is not proposed at tins time to treat of the construction of machinery for transmitting power, but to examine into the con- ditions that should determine which of the several plans of transmitting is best in certain cases whether belts, gearing, or shafts should be employed, and to note the principles upon which they operate. Existing examples do not furnish data as to the advantages of the different plans for transmitting power, because a given duty may be successfully performed by belts, gearing, or shafts even by water, air, or steam and the com- parative advantages of different means of transmission is not always an easy matter to determine. Machinery of transmission being generally a part of the fixed plant of an establishment, experiments cannot be made to insti- tute comparisons, as in the case of machines ; besides, there are special or local considerations such as noise, danger, freezing, and distance to be taken into account, which prevent any rules of general application. Yet in every case it may be assumed that some particular plan of transmitting power is better than any other, and that plan can best be determined by studying, first, the principles of different kinds of mechanism and its adaptation to the special conditions that exist ; and secondly, precedents or examples. A leading principle in machinery of transmission that more than- any other furnishes data for strength and proper propor- tions is, that the stress upon the machinery, whatever it may be, is inverse as the speed at which it moves. For example, a belt two inches wide, moving one thousand feet a minute, will theoretically perform the same work that one ten inches wide will do, moving at a speed of two hundred feet a minute ; or a shaft making two hundred revolutions a minute will transmit four times as much power as a shaft making but fifty revolu- tions in the same time, the torsional strain being the same in both cases. This proposition argues the expediency of reducing the pro- portions of mill gearing and increasing its speed, a change which has gradually been going on for fifty years past ; but there are opposing conditions which make a limit in this direction, such as the speed at which bearing surfaces may run, centrifugal strain, jar, and vibration. The object is to fix upon a point between what high speed, light weight, cheapness of cost suggest, and what the conditions of practical use and endurance demand. 44 WORKSHOP MANIPULATION. (1.) "What does the term "machinery of transmission" include, as applied in common use 1 (2.) Why cannot direct comparisons be made "between shafts, belts, and gearing? (3.) Define the relation between speed and strain in machinery of transmission. (4.) What are the principal conditions which limit the speed of shafts ? CHAPTER XL SHAFTS FOR TRANSMITTING POWER. THERE is no use in entering upon detailed explanations of what a learner has before him. Shafts are seen wherever there is machinery ; it is easy to see the extent to which they are employed to transmit power, and the usual manner of arranging them. Various text-books afford data for determining the amount of torsional strain that shafts of a given diameter will bear ; explain that their capacity to resist torsional strain is as the cube of the diameter, and that the deflection from transverse strains is so many degrees ; with many other matters that are highly useful and proper to know. I will therefore not devote any space to these things here, but notice some of the more obscure conditions that pertain to shafts, such as are demonstrated by practical experience rather than deduced from mathematical data. What is said will apply especially to what is called line-shafting for conveying and distributing power in machine-shops and other manufacturing establishments. The following propositions in reference to shafts will assist in under- standing what is to follow : 1. The strength of shafts is governed by their size and the arrangement of their supports. 2. The capacity of shafts is governed by their strength and the speed at which they run taken together. 3. The strains to which shafts are subjected are the torsional strain of transmission, transverse strain from belts and wheels, and strains from accidents, such as the winding of belts. 4. The speed at which shafts should run is governed by their size, the nature of the machinery to be driven, and the kind of bearings in which they are supported. 5. As the strength of shafts is determined by their size, and SHAFTS FOR TRANSMITTING POWER. 45 their size fixed by ike strains to which they are subjected, strains are first to be considered. There were three kinds of strain mentioned torsional, deflec- tive, and accidental. To meet these several strains the same means have to be provided, which is a sufficient size and strength to resist them hence it is useless to consider each of these dif- ferent strains separately. If we know which of the three is greatest, and provide for that, the rest, of course, may be dis- regarded. This, in practice, is found to be accidental strains to which shafts are in ordinary use subjected, and they are usually made, in point of strength, far in excess of any standard that would be fixed by either torsional or transverse strain due to the regular duty performed. This brings us back to the old proposition, that for structures which do not involve motion, mathematical data will furnish dimensions ; but the same rule will not apply in machinery. To follow the proportions for shafts that would be furnished by pure mathematical data would in nearly all cases lead to error. Experience has demonstrated that for ordinary cases, where power is transmitted and applied with tolerable regularity, a shaft three inches in diameter, making one hundred and fifty revolutions a minute, its bearings three to four diameters in length, and placed ten feet apart, will safely transmit fifty horse- power. By assuming this or any other well-proved example, and estimat- ing larger or smaller shafts by keeping their diameters as the cube root of the power to be transmitted, the distance between bearings as the diameter, and the speed inverse as the diameter, the reader will find his calculations to agree approximately with the modern practice of our best engineers. This is not men- tioned to give proportions for shafts, so much as to call atten- tion to accidental strains, such as winding belts, and to call attention to a marked discrepancy between actual practice and such proportions as would be given by what has been called the measured or determinable strains to which shafts are subjected. As a means for transmitting power, shafts afford the very important advantage that power can be easily taken off at any point throughout their length, by means of pulleys or gear- ing, also in forming a positive connection between the motive- power and machines, or between the different parts of machines. 46 WORKSHOP MANIPULATION, The capacity of shafts in resisting torsional strain is as the cube of their diameter, and the amount of torsional deflection in shafts is as their length. The torsional capacity being based upon the diameter, often leads to the construction of what may be termed diminishing shafts, lines in which the diameter of the several sections are diminished as the distance from the driving power increases, and as the duty to be performed becomes less. This plan of arranging line shafting has been and is yet quite com- mon, but certainly was never arrived at by careful observation. Almost every plan of construction has both advantages and dis- advantages, and the best means of determining the excess of either, in any case, is to first arrive at all the conditions as near as possible, then form a " trial balance," putting the advantages on one side and the disadvantages on the other, and footing up the sums for comparison. Dealing with this matter of shafts of uniform diameter and shafts of varying diameter in this way, there may be found in favour of the latter plan a little saving of material and a slight reduction of friction as advantages. The saving of material relates only to first cost, because the expense of fitting is greater in constructing shafts when the diameters of the different pieces vary; the friction, considering that the same velocity throughout must be assumed, is scarcely worth estimating. For disadvantages there is, on the other hand, a want of uni- formity in fittings that prevents their interchange from one part of a line shaft to the other a matter of great importance, as such exchanges are frequently required. A line shaft, when constructed with pieces of varying diameter, is special machinery, adapted to some particular place or duty, and not a standard product that can be regularly manufactured as a staple article by machinists, and thus afforded at a low price. Pulleys, wheels, bearings, and couplings have all to be specially pre- pared; and in case of a change, or the extension of lines of shafting, cause annoyance, and frequently no little expense, which may all be avoided by having shafts of uniform diameter. The bearings, besides being of varied strength and proportions, are generally in such cases placed at irregular inter- vals, and the lengths of the different sections of the shaft are sometimes varied to suit their diameter. With line shafts of uniform diameter, everything pertaining to the shaft such as hangers, couplings, pulleys, and bearings is interchangeable ; the pulleys, wheels, bearings, or hangers can be placed at plea- SHAFTS FOR TRANSMITTING POWER. 47 sure, or changed from one part of the shaft to another, or from one part of the works to another, as occasion may require. The first cost of a line of shafting of uniform diameter, strong enough for a particular duty, is generally less than that of a shaft con- sisting of sections varying in size. This may at first strange, but a computation of the number of supports with the expense of special fitting, will in nearly all cases saving. Attention has been called to this case as one wherein t ditions of operation obviously furnish true data to govern arrangement of machinery, instead of the determinable strains which the parts are subjected, and as a good example of the importance of studying mechanical conditions from a practical and experimental point of view. If the general diameter of a shaft is based upon the exact amount of power to be transmitted, or if the diameter of a shaft at various parts is based upon the torsional stress that would be sustained at these points, such a shaft would not only fail to meet the conditions of practical use, but would cost more by attempting such an adaptation. The regular working strain to which shafts are subjected is inversely as the speed at which they run. This becomes a strong reason in favour of arranging shafts to run at a maximum speed, provided there was nothing more than first cost to consider ; but there are other and more important conditions to be taken into account, prin- cipal among which are the required rate of movement where power is taken off to machines, and the endurance of bearings. In the case of line shafting for manufactories, if the speed varies so much from that of the first movers on machines as to require one or more intermediate or counter shafts, the expense would be very great ; on the contrary, if countershafts can be avoided, there is a great saving of belts, bearings, machinery, and obstruction. The practical limit of speed for line shafts is in a great measure dependent upon the nature of the bearings, a subject that will be treated of in another place. (1.) What kind of strains are shafts subjected to ? (2.) What deter- mines the strength of shafts in resisting transverse strain ? (3.) Why are shafts often more convenient than belts for transmitting power 1 (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 ? 48 WORKSHOP MANIPULATION. CHAPTER XII. BELTS FOR TRANSMITTING POWER. THE traction of belts upon pulleys, like that of locomotive wheels upon railways, being incapable of demonstration except by actual experience, for a long time hindered the introduction of belts as a means of transmitting motion and power except in cases when gearing or shafts could not be employed. Motion is named separately, because with many kinds of machinery that are driven at high speed such as wood machines the transmission of rapid movement must be considered as well as power, and in ordinary practice it is only by means of belts that such high speeds may be communicated from one shaft to another. The first principle to be pointed out in regard to belts, to distinguish them from shafts as a means of transmitting power, is that power is communicated by means of tensile instead of torsional strain, the power during transmission being repre- sented in the difference of tension between the driving and the slack side of belts. In the case of shafts, their length, or the distance to which they may be extended in transmitting power, is limited by torsional resistance ; and as belts are not liable to this condition, we may conclude that unless there are other difficulties to be contended with, belts are more suitable . than shafts for transmitting power throughout long distances. Belts suffer resistance from the air and from friction in the bear- ings of supporting pulleys, which are necessary in long horizontal belts ; with these exceptions they are capable of moving at a very high rate of speed, and transmitting power without appreci- able loss. Following this proposition into modern engineering examples, we find how practice has gradually conformed to what these properties in belts suggest. Wire and other ropes of small diameter, to avoid air friction, and allowed to droop in low curves to avoid too many supporting pulleys, are now in many cases employed for transmitting power through long distances, as at Schaffhausen, in Germany. This system has been very success- fully applied in some cases for distributing power in large manu- facturing establishments. Belts, among which are included all BELTS FOR TRANSMITTING POWER, 49 flexible bands, do not afford the same facilities for taking off power at different points as shafts, but have advantages in transmitting power to portable machinery, when power is to be taken off at movable points, as in the case of portable travel- ling cranes, machines, and so on. An interesting example in the use of belts for communicating power to movable machinery is furnished by the travelling cranes of Mr Ramsbottom, in the shops of the L.