Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/804

Rh 772 MECHANICS [APPLIED MECHANICS. They are usually worked by manual labour, but sometimes by hydraulic engines, or by steam-engines. The useful resistance, when a load is lifted, being the weight of that load, is in general greater than the effort exerted by the prime mover, so that the mechanism has to be adapted to giving the working-piece a less velocity than the piece to which the effort is applied. In lowering solid loads the weight of the load acts as the effort, and the energy exerted by it is expended in overcoming the friction of a brake in order that the speed of descent may not be excessive. 146. (B.) Transporting Machines. The mechanism of transport ing machines consists of two parts : that by which the resistance is diminished, as the wheels and axles of vehicles; and that by which the resistance is overcome and the load propelled, comprising all kinds of locomotive and propelling machinery. In the present work transporting machines are treated of in the articles relating specially to the lines of conveyance to which they are applied. 147. (C. ) Machines for Projecting Solids. This class comprehends all kinds of artillery. 148. (D.) Machines for Lifting Fluids. See HYDROMECHANICS. 149. (E.) Machines for Propelling or Projecting Fluids. Seethe same article. 150. (F.) Machines for Dividing Bodies. This class comprehends all machines for separating solid masses into parts, whether by digging, cutting, sawing, grinding, tearing, crushing, pounding, pressing out fluids, or otherwise ; and whether applied to earth, stones, metals, timber, fruit, grain, fibres, or other materials. 151. (G. ) Machines for Shaping Bodies by Removing Portions of them. This class of machines to a certain extent resembles the pre ceding. It includes machines for cutting, grinding, and polishing blocks of stone into required figures, and for shaping pieces of wood, metal, or other material, whether by turning, to produce spherical, cylindrical, and other curved surfaces, by boring, punching, slotting, or gouging, to produce cylindrical, rectangular, or other orifices and grooves, by screw -cutting, by planing, by grinding and polishing, to produce curved or plane surfaces. The most difficult and import ant of all these operations is to produce a surface truly plane ; and the perfecting of this operation by Whit worth is the most im portant step recently made in Constructive Mechanics, or the art of making machines and instruments. Next in point of difficulty may be placed the art of forming the concave reflecting surfaces of great specula for telescopes, such as those of the Herschels, of Lassell, and of Lord Rosse. 152. (H. ) Machines for Shaping Bodies by Pressure comprehend, amongst others, rolling-mills for iron, steam-hammers, wire drawing machines, pinmaking and nailmakiny machines, coining and other stamping machinery, brickmaking machines, presses for packing and compressing, &o. 153. (I.) Machines for Uniting Bodies into Fabrics comprise spinning machinery, whether applied to ropes, yarn, or thread, weaving machinery of all kinds, papermaking machinery, felting machinery, and sewing machinery. 151. (J.) Machines for Printing are used to apply either colouring matters or matters for discharging colour to paper, cloth, and other materials. 155. (K. ) Machines for producing Sound. See ACOUSTICS and Music. 156. (L.) Miscellaneous Machines. There are numerous machines which perform processes, especially in the preparation of textile fabrics for the market, which it would be almost impossible to class. Examples of such machines will be found by referring to the articles relating to the various branches of manufacture. CHAPTER IY. APPLIED ENERGETICS, OR THEORY OF PRIME MOVERS. 157. Prime Movers in general Their Efficiency. Prime movers, or receivers of power, are those pieces or combinations of pieces of mechanism which receive motion and force directly from some natural source of energy. The point where the mechanism belong ing to the prime mover ends and that belonging to the train for modifying the force and motion begins is somewhat arbitrary ; in general, however, the mechanism belonging to the prime mover may be held to include all pieces which regulate or assist in regulating the transmission of energy from the source of energy. Thus, in the ordinary rotative steam-engine, the crank-shaft belongs to the prime moTcr, because it carries the eccentric which moves the valves and the fly-wheel which stores and restores the periodical excess of energy of the engine, and drives the governor (when there is one) which regulates the admission of steam. The useful work of the prime mover is the energy exerted by it upon that piece which it directly drives ; and the ratio which this bears to the energy exerted by the source of energy is the efficiency of the prime mover. It is often convenient to divide the prime mover into sections, and resolve its efficiency into factors, each factor being the efficiency of ne of those sections. Thus the efficiency of a steam-engine may be resolved into the following factors : Efficiency of the furnace and boiler, being the proportion of the total heat of combustion of the fuel which takes effect in heating and evaporating the water. Efficiency of the steam in driving the piston, being the proportion of the energy exerted by the steam on the piston (called the indi cated energy or power, as being measured by an indicator) to the mechanical equivalent of the heat received by the water. Efficiency of the mechanism from the piston to the crank-shaft inclu sive, being the proportion of the effective energy transmitted by the crank-shaft to the indicated energy. The product of those three factors is the efficiency of the engine as a whole. In all prime movers the loss of energy may be distinguished into two parts, one being the unavoidable effect of the circumstances under which the machine necessarily works in the case under con sideration ; the other the effect of causes which are, or may be, capable of indefinite diminution by practical improvements. Those two parts may be distinguished as necessary loss and waste. The efficiency which a prime mover would have under given circumstances if the ivaste of energy were altogether prevented, and the loss reduced to necessary loss alone, is called the maximum or the theoretical efficiency under the given circumstances. For some prime movers there is a combination of circumstances which makes the theoretical efficiency greater thnn any other com bination does. The theoretical efficiency under those circumstances is the absolute maximum efficiency. The duty of a prime mover is its useful work in some given unit of time, as a second, a minute, an hour, a day. In some cases, such as that of the work of animals, the duty can be ascertained, while the efficiency can only be inferred indirectly or conjecturally from the want of precise data as to the whole energy expended. 158. Sources of Energy Classed. The sources of energy used in practice may be classed as follows : A. Strength of men and animals. B. Weight of liquids. C. Motion of fluids. D. Heat. E. Electricity and magnetism. 159. (A.) Strength of Men and Animals. The mechanical daily duty of a man or of a beast is the product of three quantities the effort, the velocity, and the number of units of time per day during which work is continued. It is well known that for each individual man or animal there is a certain set of values of those three quantities which make their product, the daily duty, a maximum, and that any departure from those values diminishes the daily duty. Attempts have been made to represent by a formula the law of this diminution ; but they have met with imperfect success. That which agrees on the whole best with the facts is the formula of Maschek, which is as follows : let Pj be the effort, V] the velocity, and T! the time of working per day, which give the maximum daily duty, and let P, V, T, be any other set of values of those quantities ; then One consequence of this formula is, that the best time of working per day for men, and for all animals, is one-third part of a day, or eight hours, a conclusion in accordance with experience. The best effort P lf and the best velocity V ls are much less certain, the difficulty of determining their true mean values for particular species being rendered very great by the differences, not only between individuals, but between races or varieties of the same species. The following table of values is proposed by Maschek as approxi mately true : U&amp;gt; 3 Pi- v,. TI 3600 P,V,. P,ViT,. p lb. I eetper second. Hours per day. Foot-lb per sec. Foot -ft. Man 150 lb 30 2-5 8 75 2,160,000 Horse (draught) Ox coo tb 600 lb 120 120 4-0 2-5 8 8 480 300 13,824,000 8,640,000 Ass 360 lb 72? 2-.i 8 ISO 5,184,000? Mule 500 lb 100 8 3iO 10,080,000? Of the numbers in this table those for the draught horse are prob ably the most accurate. For the thoroughbred horse it is certain that the value of V a is much greater, and that of Pj much less, than for the draught horse, the effect being probably that the maximum daily duty P^Tj is nearly the. same ; but experimental data are wanting to determine these quantities with precision. The following table, chiefly extracted from the works of Poncelet and M^rin, with the addition of some results of experiments by Lieutenant David Rankine and by the author of this article, shows the daily duty of men and horses under certain specified circum stances :