M A C H I N E R Y. 



(uch a machine is very hobbling ; but the furplus of acce- 

 lerating fjrce, at the beginning ot a returning rtroKe, will not 

 make fuch a change in the motion of the machme, if we 

 conneil the fir with it ; for the accelerating momentum is 

 a determinate- qiantity : ttierefbre, if the radius of the fly 

 be great, this momentum will be attained by communicating 

 a fmil! angular m-xion to the machine. The momentum 

 of the fly IS as the fi;|uare of its radius, therefore it refifl:s ac- 

 celeration in this proportion ; and although the overplus of 

 power generates the fame momentum of rotation in the 

 whole machine as before, it makes but a fmaU and imper- 

 ceptible addition to its velocity. If the diameter of the 

 fly be doubled, the augmentation of rotation will be reduced 

 to one-fourth. Thus, by giving a rapid motion to a fmall 

 quantity of matter, the great acceleration during thfe return- 

 (troke of the pillon is prevented. This acceleration con- 

 tinues, however, during the whole of the returning ftroke, 

 and at the end of it the machine has acquired its greatell 

 velocity. Now, the working Ih-oke begins, and the over- 

 plus of power is at an end. The machine accelerates no 

 more ; but if the power is juft in equilibrium with the re- 

 liftance, it keeps the velocity which it has acquired, and is 

 ftill more accelerated during the next returning ftroke. 

 But now, at the beginning of the fubfequent working-ftroke, 

 there is an overplus of refillance, and a retardation begins 

 and continues during the whole rife of the piilon ; but it is 

 inconliderable in comparifon of what it would have been 

 without the fly : for the fly, retaining its acquired momen- 

 tum, drags forwards the relt of the machine, aiding the im- 

 pelling power of the water-wheel. It does this by all the 

 communications takin^r into each other in the oppofite direc- 

 tion ; the teeth of the intervening wheels are heard to 

 drop from their former contaft on one fide, to a contaft on 

 the other. By coniidering this procefs with attention, we 

 eafily perceive that in a few (Irokes the overplus of power, 

 during the returning ftroke, comes to be fo adjulled to the 

 efficiency, during the working ftroke, that the accelerations 

 and retardations exatlly deftroy each other, and every fuc- 

 ceeding ftroke is made with the fame velocity, and an equal 

 number of ftrokes is made in every fucceeding minute. 

 Thus the machine acquires a general uniformity with tri- 

 fling periodical inequalities. It is plain, that by fufficiently 

 enlarging either the diameter, or the weight of the fly, the 

 irregularity of the motion may be rendered as fmall as we 

 pleafe. It is much better to enlarge the diameter : this 

 preferves the friition more moderate, and the pivots wear 

 lefs. For thefe reafons, a fly is, in general, a coiifiderable 

 improvement in machinery, by equalling many exertions 

 that are naturally very irregular. Thus a man, working at 

 a common wmdlafs, exerts a very irregular prefTure on the 

 winch. In two of his pofitions in each turn, he can exert 

 a force of near feventy pounds without fatigue, but in others 

 he cannot exert above twenty-five ; nor muft he be loaded 

 with much above this in general. But if a large fly be con- 

 nedled properly with the %vindlafs, he will aft with equal 

 eafe and fpeed againft thirty or even forty pounds. 



If any permanent change fliould happen in the impelling 

 power, or in the refiftance, the fly makes no obftacle to its 

 produftion in its full effeft on the machine, and it will be 

 obferved to accelerate or retard uniformly, till a new ge- 

 neral fpeed is acquired, exaftly correfponding with this 

 new power and refiftance. Many machines include in their 

 conftruclioti movements which are equivalent with this in- 

 tentional regulation, a flour-mill for example. There is 

 another kind of regulating fly, confifting of wings whirled 

 briflcly round till the refiftance of the air prevents any great 

 acceleration. This is a very bad one for a working machine, 



for it produces its effeft by really wafting a part of the 

 moving power. Frequently it employs a very great and 

 unknown part of it, and robs the proprietor of much work 

 It fliould never be introduced into any machine employed in 

 manufaftures, except in the inftance of letting down heavy 

 weights, where a wafte or re-a6tion to povcr is the objeft. 



Some rare cafes occur where a very diflFerent regulator is 

 required, when a certain determined velocity is found ne- 

 ceflary : in this cafe, the machine is furniflicd at its extreme 

 mover with a conical pendulum, confifting of two heavy 

 balls hanging by rods, which move in very nice and fteady 

 joint at the top of a vertical axis. It is well known, that 

 when tliis axis turns round, with an angular velocity fuited 

 to the length of thofe pendulums, the time of a revolution 

 is determined. 



Thus, if the length of each pendulum be 39^ inches, the 

 axis will make a revolution in two feconds very nearly. 

 It we attempt to force it more fwiftly round, the balls 

 will recede a little from the axis, but it employs as long 

 time for a revolution as before ; and we cannot make it turn 

 fwifter, unlels the impelling power be increafed beyond all 

 probability: inwhichcaie, the pendulum will fly out from 

 the centre till the rods are horizontal, after which every 

 increafe of power will accelerate the machine very fcnfibly, a« 

 it tlien becomes a fimple fly. Watt and Boullon have applied 

 this contrivance with great ingenuity to their fteam engines 

 when they are employed for driving machinery for manu- 

 factures which have a very changeable refiftance, and where 

 a certain fpeed cannot be much departed from without great 

 inconvenience. They have connefted this recefs of the balls 

 from the axis (which gives immediate indication of an in- 

 creafe of power, or a diminution, or refiftance,) with a 

 cock, which admits the fteam to the working cylinder. 

 The balls flying out caufe the cock to clofe a little, and 

 diminifli the fupply of fteam, if the impelling power di- 

 minifties the next moment, and the balls again approach the 

 axis, and the rotation goes on as before, although ihera 

 may have occurred a very great excefs or deficiency of 

 power. The fame contrivance may be employed to raife 

 or lower the feeding fluice of a water-mill employed to 

 drive machniery. (See Mill.) Suppofe all refiftance re- 

 nioved from the working point of a machine furniflied with 

 a very large or heavy fly, immediately connetlcd with the 

 working point ; when a fmall force is ap}>lied to the im- 

 pelled point of this machine, motion will begin in the 

 machine, and the fly begin to turn, continue to prefs uni- 

 formly, and the machine will accelerate. This may be con- 

 tinued till the fly has required a very rapid motion. If, at 

 this moment, a refilling body be applied to the working 

 point, it will be afted on with very great force ; for the fly 

 has now accumulated, in its circumference, a very great mo- 

 mentum. 



If a body were expofed immediately to the a£lion of this 

 circumference it would be violently ftruck, much more will 

 it be lo, if the body be expofed to the aclion of the work- 

 ing point, which perhaps makes one turn while the fly makes 

 a hundred. It will exert a hundred times more force (very 

 nearly) than at its own circumference. All the motion wh.ch 

 has been accumulated in the fly, during the whole progrefs 

 of its acceleration, is exerted in an inftant at the working 

 point, multiplied by the momentum, which depends on the 

 jjroportion of the parts of the machine. It is thus that the 

 coining prefs performs its office ; nay, it is thus that the 

 blacklmith forges a bar of iron. Swinging the great fledge 

 hammer round his head, and urging it with force the whole 

 way, this accumulated motion is at once cxtmguiflicd by 

 impafl on the iron. It is thus vre drive a nail ; and it is 



tlius, 



44406*^ 



(•W 



