STANDARD, ETC., OF POWER.] 



APPLIED MECHANICS. 



781 



work. Whether it be an accurate expression of the 

 power of an average horse or not, is of little consequence, 

 provided it be generally accepted and understood as a 

 standard measure of power. Were there no such 

 standard, each mechanic might make an estimate of his 

 own : one might compute the work of a very strong 

 horse, the other of a very weak one. Thus, steam- 

 engines or other apparatus might be supplied of all 

 different strengths and sizes, and yet purporting to be 

 of equal powers ; or apparatus of like strengths and 

 dimensions might be stated to be of very different 

 powers. But, this standard once fixed, it is the duty of 

 a mechanic to estimate, by experiment or calculation, 

 the weight which the engine he makes can lift through a 

 given height in a given time, or the weight which moving 

 through a given height in a given time, can work 

 efficiently some apparatus which he may have fabricated ; 

 and he can then state the power which his engine 

 furnishes, or which his apparatus requires, in terms in- 

 telligible to all the world. It is clear from what has 

 preceded, that a horse-power does not mean precisely 

 33,000 Ibs. lifted one foot in one minute, but a power 

 equivalent to that ; as, for instance, 330 Ibs. lifted 100 

 feet in one minute, 3,300 Ibs. lifted one foot in one-tenth 

 of a minute ; or, in fact, any weight such that, on mul- 

 tiplying it by the distance moved and dividing the 

 product by the time occupied in the motion, the result 

 shall be 33,000. 



These considerations may be somewhat simplified by 

 combining the distance and time into one term, which 

 we call velocity or speed. Velocity is directly propor- 

 tional to the distance passed over, and inversely pro- 

 portional to the time occupied in the transit. A railway 

 train that passes over 50 miles in an hour, has double 

 the velocity of one that travels 25 miles an hour, because 

 60 is double 25. Again, a train that travels 25 miles 

 in one hour hag double the velocity of a train that passes 

 over the same distance in two hours. To compare the 

 velocities numerically, we divide the distances by the 

 times of each. 



In estimating powers, since onr standard is given in 

 feet for distance, and minutes for time, we divide the 

 distance in feet by the time in minutes, and thus get 

 the velocity. The weight in pounds Hfted, multiplied 

 by the velocity thus reckoned, gives a product which, 

 being divided by 33,000, shows the number of horses'- 

 power. Suppose an engineer were required to furnish a 

 steam-engine capable of pumping 165,000 gallons of 

 water every hour, to a height of 120 feet, he would 

 reckon thus : An hour contains 60 minutes ; and 120 

 divided by 60, gives two feet per minute as the velocity 

 with which the required volume of water must be lifted. 

 A gallon of water weighs 10 Ibs. ; therefore, 165,000 

 gallons weigh 1,650,000 Ibs. ; this weight moved at the 

 velocity of 2 feet per minute is equivalent to l,650,000x 

 2, that is 3,300,000 Ibs. lifted 1 foot in 1 minute. 

 Dividing this by 33,000, the quotient is 100 horse-power 

 as the actual force required to do the work in the time 

 given. He would, therefore, proceed to make an engine 

 which, after providing for all mechanical losses in the 

 operation, should be capable of producing this effect. 



SOURCES OF POWER. The principal sources of 

 power are, the muscular forces of men and animals, the 

 natural motions of air and water, the weight and elasticity 

 of materials, and the changes effected in bodies by the 

 action of heat, and by electrical and chemical action. It 

 is the business of the mechanic to utilise these forces, to 

 regulate and control them, to change or modify their 

 directions, velocities, or intensities, so that they may 

 be made to do certain work in the best, most economical, 

 and expeditious manner. In addition, therefore, to his 

 knowledge of the materials on which, or through which, 

 these forces have to act, he must have an intimate a.c- 

 quaintance with the nature and laws of the forces them- 

 selves, as discovered by experiment, or investigated 

 abstractly. It is a fortunate circumstance for mam, con- 

 sidering the brief period allotted for individual research, 

 that the laws of nature are of the most simple character, 

 and that he is gifted with faculties that enable him to 



communicate the results of his investigations througl 

 great distances and over lengthened periods. Every sue 

 cessive discovery in natural science throws additiona 

 light over all that has preceded, opens up new fields for 

 research, and suggests new modes of practical action 

 In the arts, likewise, an ingenious invention simplifies 

 much that formerly was rude and cumbrous, facilitate; 

 operations formerly deemed impracticable, and furnishes 

 the means of practising new and unheard-of branches 

 of art. Let one but compare the state of mechanical arl 

 as it was before the time of Watt, and as it is now ; anc 

 the more deeply he investigates the question, the more 

 will he be surprised at the enormous change effected by 

 a few simple but most ingenious modifications of 

 machine for utilising the power developed by subjecting 

 water to the action of heat. 



The steam-engine has indeed exerted so vast an in- 

 fluence on the arts, that we shall think it necessary to 

 devote considerable space in what follows to its de- 

 scription in detail. We are the more strongly urged to 

 this, because we believe that the student who can master 

 the details of the construction and application of this 

 apparatus, will be prepared to find his way through any 

 branch of Practical Mechanics. 



APPLICATION OF POWER The mere supply 

 of forces for our various purposes would be of little 

 benefit to us unless we had ingenuity sufficient to find 

 modes of applying them. All work, as we have already 

 hinted, means effecting changes of some kind or other ; 

 and all changes of matter imply motions either of masses 

 or of parts. A most interesting and extensive branch 

 of Practical Mechanics consists, therefore, in the dis- 

 cussion of the various modes of communicating and 

 modifying motion. The two elements of which power 

 consists viz. , the weight or mass acting, and the velocity 

 with which it acts are convertible terms. Having a 

 certain power to work with, we can give up a certain 

 amount of weight, and thereby gain in velocity, or 

 sacrifice velocity in order to gain in weight. We cannot 

 create any addition to the power either in weight or in 

 velocity, nor can we annihilate any portion of it. We 

 can, doubtless, apply the power with better effect to the 

 work, and thereby save a loss ; or we can misapply a 

 portion of the power, and thereby render it ineffective 

 for the purpose intended. A power applied to put in 

 motion any part of a train of machinery, will be found 

 acting at any other part of the same train. 



If we could make our workmanship absolutely perfect, 

 could have materials absolutely rigid and frictionless, 

 and could remove our machinery from the influence 

 of all extraneous existence, we should find in every part 

 of it an equal amount of power. Some parts may move 

 more slowly than others, but then they act with greater 

 pressure or weight ; some parts may act with less pres- 

 sure, but then they move with greater velocity : in short, 

 in every part of such a perfect macliine we should find 

 the pressure multiplied by the velocity that is, the 

 power or momentum, exactly the same. Even with our 

 comparatively imperfect workmanship, we are quite safe 

 in estimating according to this rule, without making 

 much allowance for extraneous resistance. In the works 

 of a clock, the wheel fixed to the barrel which carries 

 the weight revolves so slowly, that its motion is quite 

 imperceptible ; and through the teeth of this wheel is 

 conveyed the force which puts the whole clock-train in 

 motion, a weight of many pounds. Again, the escape- 

 ment-wheel, which sustains the motion of the pendulum, 

 revolves with comparative rapidity, but with so little 

 apparent power, tliat an opposing pressure of a few 

 ounces might completely stop it ; yet, on stopping the 

 escapement-wheel, we stop also the barrel-wheel, and 

 the few ounces applied to the one thus effectually oppose 

 the many pounds acting on the other : in fact, the 

 weight which acts on the barrel, multiplied by the velocity 

 with which it moves, is equal to the weight necessary to 

 stop the escapement, multiplied by the velocity with 

 which it moves. 



FRICTION. We have alluded to resistances in ma- 

 chinery, the principal of which is friction, or the retarda- 



