A I'! 1 LIED MECHANICS. [MODES OF COMMUNICATING POWER. 



blade* to the plane of tho circle, and thii obliquity in- 

 erea*e* from tho c-iroiimferenoe towards the centre, be- 

 cause the part* nearer the centre travel with less cir- 

 cumferential Telocity, and yet the water passes them 

 with o<|ual longitudinal speed. When we say that a 

 screw has twenty-feet pitch, we mean that if the screw- 

 blade were continued through a complete revolution 

 round its axis, any straight line drawn parallel to the 

 axis would c', '.HUM of the blado at points twenty 



feet apart. Did the screw, in revolving through the 

 water, put the latter in motion at such a rate as pre- 

 cisely to glide along its blades in straight linos parallel 

 to the axis, every revolution of the screw would move 

 any portion of water exposed to it through a length of 

 20 feet. But, practically, the water is almost at rest, 

 and the screw worms its way through it, having to put 

 in motion the vessel to which it is connected, and a 

 certain tlip occurs, or the speed with which the screw 

 and vessel travel through tho water is less than that duo 

 to the obliquity of the screw by about 25 or 30 per cent. 

 If wo suppose that a screw of 20 feet pitch makes 00 

 revolutions per minute, any point in its surface without 

 slip would move through 20 X 60 = 1200 feet longi- 

 tudinally per minute, or 1200 X 60 = 72000 feet = 12 

 knots per hour nearly. But, allowing twenty-five per 



cent., or one-fourth, for slip, the actual speed would bo 

 nine knots per hour. 



[TuB STKAM HAMMER. Of late years, the immense 

 domand for heavy forgings has necessitated novel 

 methods for reducing them to the proper shape, espe- 

 cially when required in connection with screw-propellers, 

 one of the largest of which was that ustxl in tho ' 

 Etuteru. The steam hammer has been much emp' 

 for this purpose ; and in the hands of Nasmyth, Coin lie, 

 and others, has been brought to a high degree of perfec- 

 tion. The general principle of construction is that of a 



solid vertical piston, vod in a cylinder by steam ; the 



motion being regulated in a similar manner to that of 

 the ordinary steam-engine, by tho action of a slide-valve 

 moved by an attendant. The piston-rod is extended, so 

 as to form the hammer-head ; and below, on a sur 

 block, the iron to be forged is placed. The admission of 

 steam beneath the piston raises it, and tho hammer-head 

 can then be driven, so as to impinge directly, at the ex- 

 tremity of tho rod, on the article to be forged. By 

 regulating the admission of steam, any amount of 

 power may be exerted a fact of great value to the 

 practical engineer. Tho folio plate represents a machine 

 of this kino, which was shown in action in tho Exhi- 

 bition at London, in 1862. ED.] 



CHAPTER VII. 



THE COMMUNICATION OF POWER. 



Content*. WORK AND POWER ROTARY MOTION COUPLINGS CLUTCHES PLUMMER-BLOCKS PULLEYS, OR DRUMS 



TOOTHED WHEELS FORMS OF TEETH INVOLUTE OF A CIRCLE BEVIL GEAR RACKS WORM AND WIIKKL 



XPICYCLOIDAL TEETH RECIPROCATING MOTION DISCONTINUOUS MOTION RATCHET CAM MANGLE-MOTION 



REVERSING GEAR SCREW-CUTTING GEAR ELLIPTICAL GEAR SUN AND PLANET FRICTION BREAKS DYNA- 

 MOMETER BREAKS RESISTANCE OF FRICTION FRICTION OF STRAPS MOTION OF FLUIDS. 



WORK AND POWER. The communication of power 

 is really the communication of motion, for power implies 

 change, and change of place is motion. A column sup- 

 porting a weight communicates the pressure of the 

 weight to its foundation ; and a string suspending a load 

 from a hook, communicates its tension to tho hook ; but 

 in neither of these cases is power communicated, for no 

 change is effected : time forms no element in tho ques- 

 tion, the same strain being conveyed to tho point of 

 support in an instant, as in a century. But if tho 

 column or string be in motion, like the piston-rod of a 

 team-engine, pushing or pulling against a resisting 

 force, not only is tho pressure acting on the piston cou- 

 reyed through the rod as a simple strain, but it is also 

 conveyed through a certain distance in a certain time, or 

 at a certain velocity, and can, by its motion, effect 

 changes on materials presented to it, proportioned to the 

 amount of power developed, and the time during which 

 it acts. The power of any mechanical arrangement, 

 therefore, meansits capability of effecting change ; while 

 it* icork means the quantity of change effected. The 

 most simple kind of change wlu'ch we can contemplate is 

 that of position ; and we, therefore, take change of 

 position or motion as the measure of one element of 

 work dona Tho most simple notion as to quantity of 

 change, is that of mass or weight of material moved ; 

 and we therefore take weight as tho measure of another 

 of work. Lastly, we estimate the capability of 

 , change that is to say, power by the time re- ,0' 

 quired for the work done. Tho greater the mass niov. .1. 

 the greater the distance over which it is moved ; and the 

 less the time occupied in the motion, the greater the 

 power developed. 



An engine of 10 horse-power, means an engine capable 

 of moving 10 times tho mass that can bo moved by an 

 engine of 1 hone-power over 11 given distance in a given 

 time, or of moving the same mass over 10 times the 

 distance iff the same time, nr of moving the same mass 

 .v. r the tamo distance in ^th of the time. Power, 

 indeed, is simply pressure multiplied by velocity ; and 

 if we know the pressures and velocities communicated 



through any trains of machinery, we compare their 

 powers by comparing th< products of these factors. 



It is a simple law of mechanic*, admitting of no ex- 

 ception, that, whatever be the nature or complication of 

 any mechanical arrangement by which power is conveyed, 

 whatever be the power applied to more it, the same 

 power would bo given out from every part of it, provided 

 no waste occurred from friction. And tho more perfect 

 the construction of tho machinery in respect of smooth- 

 ness of rubbing surfaces, tho moro nearly is this law 

 practically fulfilled. 



Fig. 211. 



If we take any of the simple mechanical powers, such 

 as tho lever A B (Fig. 212), capable of vibrating on tho 

 fulcrum F, wo observe that when A F is equal in length 

 to F B, a weight of 10 Ibs. hung from A, balances the 

 same weight hung from B ; or, when A F is twice F C, 

 a weight of 10 Ibs. at A balances 20 Ibs. at C. 



Such is the law for moro pressure or weight without 

 motion ; but when we consider the question in respect of 

 power, we have to imagine that tho lever vibrates on its 

 fulcrum, so that every point of it describes a portion of 

 a circle round the centre F. Wo may suppose one such 



