M A C H I N E. 



jolts to the machine, and great flrains on its moving parts, 

 and their points of fupport ; whereas, when they are gra- 

 dually lifted, the inequality of the motion is never felt at 

 tiiat point of the machine where the power is applied. We 

 have fcen, fays the profeffor, pi (Ions moved by means of a 

 double rack on the' pifton rod, where a half wheel takes 

 hold of one rack, and raifes it to the required height ; and 

 the moment the half wheel has quitted that fide of the rack, 

 it lays hold of the other fide, and forces the pifton down 

 again. This was cotilidcred as an improvement of the com- 

 mon method of the crank, by correctin^^ the uiieqiiable mo- 

 lion of the pifton. But in fail it is far inferior to the latter, 

 as it occafions fuch abrupt chani^es of motion, that the ma- 

 chine is ftiaken and torn to pieces with the jolts it occafions ; 

 a circumftance which will always be avoided as much as 

 pofTible by a judicious engineer. 



When feveral ftampers, pifton?, or other reciprocal 

 movers, are to be raifed and deprefted, their times of ac- 

 tion ought to be diftributed in a uniform manner, fo that 

 the machine may sal ways be equally loaded with work. 

 When this is done, and the obfervations in the preceding 

 paragraph attended to, the machine may be made to move 

 almoft as fmoothly as if there were no reciprocations on it. 

 Nothing {hews the ingenuity of the conftruftor more than 

 the artful, yet fimple, contrivances for obviating thofe dif- 

 ficulties- that unavoidably arile from the very nature of the 

 work that muft be performed by the machine, and of the 

 power employed. We mentioned, above, the converfion of 

 the continued rotation of an axis into the reciprocating mo- 

 tion cf a pillon, and the improvement that was thought ..to 

 have been made in the common and obvious contrivance of 

 the crank, but which, as was obferved, occafioned fuch 

 jolts as would in a fhorl time have dellroyed the machine. 

 In order to avoid this, in a large forge where a great fledge 

 hammer of feven hundred weight was to be railed, the en- 

 gineer formed the wipers into fpirals, which communicated 

 motion to the hammer almoft; without any jolt whatever : 

 and under fome circumftances, this contrivance would have 

 been highly beneficial ; but in the machine to which we al- 

 lude, it would not apply, as it did not communicate a fuf- 

 ficient momentum to the hammer in its defcent : yet it is 

 deferving of notice, as it might in fome cafes become ex- 

 tremely advantageous. 



In employing a power, which of neceffitv reciprocates, to 

 drive machinery, in which a rotatory motion is required, as 

 in applying the ftcam-engine to a cotton or grirt-mill, C9n- 

 fiderable difficulties alfo arife, which muft be attended to 

 with particular care. The neceftlty'of reciprocation on the 

 firft mover wattes much power, becaufe the iullruincnt that 

 communicates fuch immenfe force muft be extremely ftrong, 

 and well fupported. The impelling power is wafted in im- 

 parting, and afterwards deftroying, a great quantity of 

 motion in the working beam. The fkilfiil engineer will at- 

 tend to this, and do his utmoft to procure the neceftary 

 ftrength of the firft mover, without miking it a vail load of 

 inert matter : he will alfo ren:iark that all the ftrains on it, 

 and on its fupports, are changing their direction on every 

 llroke ; which therefore requires particular attention in the 

 manner of fupporting it. It we obfcrve ftcam-engines that 

 have been long erected, we ftiall find that they have uni- 

 formly fhaken the building to pieces, which is principally 

 to be attributed to the inattention of the engineer to this cir- 

 cumftance ; and experience has now taught us, that no 

 building can long vvithftand the defukory and oppofite jolts 

 of Inch immenfe maifes ; and, cenfequently, that the great 

 movements ought to be lupported by a frame-work, inde- 

 pendent of the building which coniains it. Another cir- 



cumftance, on which the uniformity of the motion dep'cndf, 

 is the form given to the teeth of the wheel : this is of great 

 importance, and has excited great attention amongft both 

 theoretical and praftical machinicians. Two forms have 

 been propofed : of thefe the firft was given by La Hire, 

 who affirmed that the preffiire would be uniform, if the 

 teeth were formed into epicycloids ; and M. Camus, in his 

 " Cours de Mathematique," has' adopted and purfucd La 

 Hire's principle, and applied it to the various cafes that are 

 likely to arife in practice. This conftrudlion, however,^ is 

 liable to a limitation ; on which account, a fecond method 

 has been propofed, which fecures the perfect uniformity of 

 motion, without any fuch limitation. This confifts of mak- 

 ing both teeth portions of involutes of circles; but as wc 

 Ihall confider this fubjeft under the articles Tooth and 

 Pinion Work, it will be ufelefs to infift any farther upon it 

 in this place ; and we will therefore proceed to the theo- 

 retical inveftigation of the power of machines, and their 

 maxiiiium cffefts ; limiting our obfervation to thofe prin- 

 cipally whofe motion is uniform, thefe forming by far the 

 moft; numerous clafs, and the knowledge of which is, there- 

 fore, of the greatell importance. 



Ofthi mcix-imiim EJecls of Machines When forces afting 



in contrary directions, or in any fuch diredlio;is as produce 

 contrary effefts, there is with refpeft to every fimple ma- 

 chine, and confequently with refpeiit to every compound 

 one, a certain relation between the powers and the diftances 

 at which tliey aft, which, if fubfifting in any fuch machine 

 when at reft, will prcferve it in that ftate of ftatical equi- 

 hbriunv; becaufe the efforts of thefe powers, when thus 

 related with regard to magnitude and diftance, being equal 

 and oppofite, deftroy each other, and have no tendency to 

 change the ftate of the fyftem to which they are applied. 

 So alfo, if the fame machine have been put into a ftate of 

 uniform motion, whether reililinear or rotatory, by the 

 action of any power diftinft fi-om thofe we are now confi- 

 dering, and thefe two powers be made to act upon the ma- 

 chine in fuch motion, in a fimilar manner to that in which 

 they aCt upon it when at reft, their fimultancous aftion 

 will preferve it in that ftate of uniform motion, or dynamical 

 equilibrium, and this for the fame reafon as before ; becaufe 

 thfir contrary effefts deftroy each other, and have, there- " 

 fore, no tendency to change the ftate of the machine. But 

 if at the time a machi.e is in a ftate of balanced reft, anvv 

 one of the oppofite forces be increafed, while it continues 

 to aft at the lame diftance, this excefs of force will difturb 

 the ftatical equilibrium, and produce motion in the machine; 

 and if the fume excefs of force continues to act in the fame 

 manner, it will, like every conftant force, produce an ac- 

 celerated motion ; or if it fhould undergo particular modi- 

 fications, whea the machine is in ditTerent pofitions, it may 

 occafion fuch variations as will render it alternately acce- 

 lerated and retarded. Or, the different fpecies of reiiftancc 

 to which a moving machine is fubjefted, as the rigidity of 

 cords, friction, refiftance of the an-, &c. may fo modify it, 

 as to change a regular or irregular variable motion into 

 one which is uniform. Hence, then, the motion of machines 

 m^y be confidered as of three kinds, as that which is gra- 

 dually accelerated, which obtains commonly in the firll in- 

 ftants of the communication. i. That which is entirely 

 uniform. 3. That which is alternately accelerated and re- 

 tarded. Pendulum clocks and machines that are moved bv a 

 balance are related to the third clafs. Moft other machines 

 are of the iecond clafs, at leaft a fhort time after their mo- 

 tion is commenced. 



Nov.-, although trie motion of a machine be alternately 



accelerated and retarded, it may, notwithllanding, be mea- 



j B 2 furej 



