I\I A C H I N E. 



commodate tlio direction of the moving force to that of the 

 re liihince to be overcome : to incrcafc the cfled of a given 

 fmite power, fo as to overcome a refiilance which is greater, 

 and would olherwifc ever remain unchanged : and lallly, to 

 regulate and modify a variable force, fo as to produce a con- 

 ftant and uniform elTec^. Thefe are the principal endsto te 

 accomplifhed by machines, and tha experienced engineer 

 will always endeavour to execute them in the fimplell man- 

 ner poffible; for complicated machinery is not only mod 

 liable to inaccurate adjudment, and frequent difanangement, 

 but is likewife more cumberfome and cxpeufivc, at tlie fame 

 time that the retardation arifmg from fridion, adhefion, and 

 inertia, is more confiderable, and conftquently a greater 

 power becomes neceil'ary," in order to produce the fame 

 effeft. Another important point to be attended to, is the 

 molt advantageous ajiplication of the firll mover, whether 

 this agent be air, water, Ream, or animal ftrength. To 

 enter upon this qiiellion in all its generality, would far ex- 

 ceed our limits; bifides, with regard to the three former, they 

 \vill be better invelligated under the articles Wind and Water 

 Mills, ^t-exm Engine, &c.; what few remarks, therefore, 

 we have to make on this head, will be confined to the appli- 

 cation of animal exeriion to the motion of machines, and for 

 the other agents we mult refer the reader to the articles 

 above-mentioned. 



We have aftriking inllancc of the injudicious application 

 of the exertion of men, in the old crane worked by means ot 

 an internal walking wheel, which, from its nature, mult be 

 very heavy, while the action of the man is exerted at a very 

 trifling diftar.ce from the axle, and confequently at a great 

 mechanical difadvantage ; whereas in Hardie's crane, the 

 ir.an acting externally at the greatefl; diltance from the ful- 

 crum, produces a much greater effedl with lefs expence of 

 labour ; the other advantages which this machine poffefTes 

 over the one above-mentioned, not arifmg lolely from this 

 caufe, are not conneAed with our preient enquiry. 



The above remark applies principally to the mechanical 

 advantage to be obtnined in the application of a firil mover ; 

 but there is alfo another confideration of a phyfical nature, 

 which is equally important, and ought therefore to be par- 

 ticularly attended to. No animal can exert more than a de- 

 terminate and limited force; and, confequently, if it re- 

 quires all this force merely to produce an equilibrium, no 

 effeft will refult from the aftion : and, on the other hand, 

 if all the ftrength of a man or horfe is employed in giving 

 motion to himfelf, or to external objcfts before the applica- 

 tion reaches the refiilance, there is Hill the fame unproduc- 

 tive eP.ed. A man, for exam;;lc, pnfliing at a capltan bar, 

 muft fird of all walk as faft as the bar moves round, which 

 evidently requires an expenditure of his mufcular power ; 

 but this alone will not render his exertion effective : tie muil 

 alfo prefs the bar forward, with as much force as he has re- 

 maining above that which he expended in walking at that 

 ra'e. The proportion of thefe two expenditures may be 

 very different under diflerent circumllances ; and on the 

 judicious feleftion of fuch circumftanees as make the firll of 

 thefe aS fmall as poffible, lies much of the IkiU of the en- 

 gineer.. In the common operation of thrafhing corn, much 

 more than half the man's power is expended in giving the 

 necefTary motion to his own body ; and only the rfcmaindcr 

 is employed in urging forward the fwiple with a momentum 

 fufficient for fhakmg out the ripe grains from the ilalk. ' Dr. 

 Robifon mentions an experiment, made in order to afccrtam 

 the quantity of power thus loll. In order to which, the 

 fwiple was taken off the flail, and the fame weight of lead 

 put on the end of the ftaff ; then by caufing the labourer to 

 perform tlie ufual motions of thrafhing, with all the rapidity 



that he could continue during the ordinary hours of work ; 

 it was found that the number of iiK^tions thus made was 

 to thofe made in the actual operation of thrafhing, in about 

 the ratio of j toa : whence we may inter, tlmt at leall half 

 the tliraflier's power is expended in merely moving his own 

 body. We may alfo bring another very fimplo cafe, by way 

 of further illullration. 8uppofe a quantity of earth is to be 

 removed from one place to another by barrows. It is ob- 

 vious tliat the loads may be fo great, that a man mull exert 

 his whole ftrength barely to lift up the fliafts, and confe- 

 quently will have none left to pufh the barrow forward : if 

 part of the load be taken off, he can go forward, and fu 

 much the failer as tlie quantity of the load is reduced ; but 

 if even the whole be taken away, he can -dill only move at a 

 certain rate, and, confequently, in neither of the extreme 

 cafes isfany effeit produced. It becomes then an intereding 

 quedion to determine what load he ought to carry, in order 

 to produce the greated pofiible efTedt in a given time. We 

 fliall not, in this place, enter any, farther upon this fubjeft, 

 truding that what has been already advanced will be lufTi- 

 cient to point out the ncceffity of attending to fuch circum- 

 llances ; and in the fubfequent part of the preient article, 

 we will endeavour to explain in what manner the proper ad- 

 judment of power and effett may be computed. 



The nature of the lirfl movement being determined, the 

 next object is to communicate it_ to the dtllined point, where 

 the refinance is to be overcome ; and much of an artid's ikill 

 depends upon performing this in the fimplefl and mod cf- 

 fedual manner pofnble. In ord.er to this, it frequently be- 

 comes necefTary to convert one fpecies of motion into an- 

 other fpecies : as, for exatnp'e, a rotatory into a recipro- 

 cating motion, or a reciprocating into a rotatory motion, 

 Sec. &c. The methods of forming this communication are 

 extremely numerous, and it will not therefore be expeded 

 that we fhould attempt an enumeration of them. In fome 

 indauces, a fimple Jever or tinbent cord will anfwer better 

 than any combination ; in others, it is highly advantageous 

 to ufe a combination of levers ading upon each tilher by 

 means of fo many fulcra, and by which the diredion of the 

 motion may be changed at pleafure ; in others, as when mo- 

 tion is communicated to a feries of wheels and axles, in iuc- 

 cefiion, it may be cffeded by a rope running in grooves 

 round one wheel and the fucceeding axle, or by means of 

 tooth and pinion work, by a barrel and endlefs fcrew, and 

 various other contrivances which will naturally fugged them- 

 felves, according to the circumitances under which they 

 arife. 



This part of the condrudion being fettled, other im- 

 portant circumdances require particular attention, iiix,. to 

 adjud the feveral parts of the machine fo, that its motion 

 may be eafy, free, and uniform. One of the mod obvious 

 methods of rendering a motion uniform is by means of a 

 J pendulum zn& fcapcment (fee thefe two articles) ; and where 

 thefe cannot conveniently apply, a fly is fon.etimes em- 

 ployed ; for a particu'ar defcription of which, fee Fly. 

 The uniformity of a machine is not, however, wholly de- 

 pendent upon the application of fuch regulators : there are 

 other points conneited with this fubjed, that mull not be 

 overlooked, and on which we intend to offer a few remarks ; 

 ■availing ourfelves, for this purpofe, of the obfervations of 

 Dr. Robifon. When heavy dampers are to be ratfcd, in 

 order to drop on the matters to be pounded, the wipers by 

 which they are lifted fhouid be made of fuch a form, that 

 the damper may be raifed by a uniform preffure, or with a 

 motion almoft perfedly uniform : if this is not attended to, 

 and the wiper is merely a pin dieking out from the axi?, the 

 damper is forced into adion at once, which occafions violcHt 



jolts 



