MACHINERY. 



for another load, which alfo muft be done gradually. All 

 this waftcs a great deal both of time and force, and renders 

 a walking-wheel a very improper form for the firll mover of 

 a crane, or any machine whofe ufe requires fuch frequent 

 changes of motion. The fame thing obtains, although in a 

 lower degree, in the (team-engine, where the great beam 

 and pump-rods, fometimes weighing many tons, mufl be 

 made to acquire a vorv bviflc motion in oppofite diredlions, 

 twice in every working ttrokc. It operates in a greater or 

 a Icfs degree, in all engines which have a reciprocating mo- 

 tion in any of their parts. Pump-milb are of neceffity fiib- 

 jefted to this inconvenience. In the famous engine at 

 Marly, about i-ths of the whole moving power of fomc 

 of the water-wheels ia employed in giving a reciprocating 

 motion to a fet of rods and chains, which extend from the 

 wheels to a ciftern about three-fourths of a mile diftant, 

 where they work a fet of pumps : thus the engine is, by 

 fuch injudicious conftruflion, a monument of magnificence, 

 &nd the druggie of ignorance with the unchangeable laws of 

 nature. In machines, all the parts of which continue the 

 diredlion of their motion unchanged, the iner'.ia of a great 

 mafs of matter does no harm ; but, on the contrary, con- 

 tributes to pfeferve the fleadincfs of the motion, in fpite of 

 fmall inequalities of power or refiftance, or unavoidable ir- 

 regularities of force in the interior part. But in all recipro- 

 cations, it is highly prejudicial to the performance ; and, 

 therefore, conftrudtions which admit fuch reciprocation 

 without neceffity, are avoided by all the intelligent en- 

 gineers. 



In many machines, but generally in fmall works, what 

 are called hearts, camms, fnails, excentric wheels, &c. are 

 a very excellent method of producing flight reciprocating 

 movements to levers. From the rotatory motion of an axis, 

 they have the great advantage of admitting any modification 

 of the motion, to aft fuddenly or gradually, in either direftion, 

 at the pleafurp of the maker. This is done, by wheels of a 

 particular form, fattened upon an axis, and levers applied in 

 contaft with their circumferences, which receive a motion in 

 proportion as the different radii of the wheels alter their 

 lengths ; and if, at any point of the motion, the lever is to 

 be in a ftate of reft, the periphery of the wheel is, during 

 that period, made a circular arc, and concentric with the 

 axis. From the facility of producing any motion whatever 

 by carams, it is an univerfal method, and applicable to all 

 fubjefts ; but ftill has objeftions, which will induce the en- 

 gineer to negleft it in thofe mftances, where any other move- 

 ment will anfwer the fame purpofe. Thefe objeftions are 

 the great friftio;i, and wear of the camms, which foon unfits 

 them for accurate motion ; this may in i'ome meafure be ob- 

 viated by applying rollers in the ends of the levers, to re- 

 . ceive the contaft of the camm. Another objeftion is, that 

 the camm is unfit for producing a double motion, becauie a 

 fpring or weight muft be introduced to return the lever, and 

 always keep it in contaft with the camm. Now if this 

 fpring is only ufed to return the lever, it will operate very 

 well ; bu' if it is made fo ftrong as to effeft any operation of 

 the machine, the friftion will be grea', and be a ferious ob- 

 jeftion to the ufe of camms. 



The principles of thefe movements, and praftical direc- 

 tions for conftrufting camms for any kind of movement, is 

 fully explained in our article Diagonal Motion, which 

 renders it unneceflfary to enlarge upon the fubjeft in this 

 place. Camm.s are ufed on a large fcale in rolling-mills, for 

 working the flicars with which large iron bars are clipped 

 into lengths. They are alfo employed in the machine for 

 punching holes through the iron plates for boilers, weaving 

 Tnachmes, &c. ; and are in common ufe in the blowing ma- 



chine ufed in iron forges ; but it is a very injudicious ap- 

 plication, and a common crank would be much better. 



We once with great plcafure contemplated a very com- 

 plicated machine, m which were many reciprocating parts 

 necefiarily operating only whilil moving in one direftion ; in 

 the other, they had merely to return to repeat their opera- 

 tions. To produce this reciprocation, the inventor applied 

 a crank, which was caufed to revolve by theaftion of a pair 

 of elliptical cog-wheels, each balanced on an axis pafiing 

 through one of its foci. In this conllruftion, the motion 

 of the driven wheel and the crank it carried, was exceedingly 

 variable, but by equal increments of alternate acceleraion 

 and retardation. Thus when the long radius of the lirft 

 wheel was operating, it met the (hortell radius of the other, 

 therelore giving it and alfo the crank a rapid motion : in 

 this ftate, the crank was returning to repeat its ftroke, and 

 with a quick ftroke ; but by the time it had completed half 

 a revolution, the aftion was reverfed, the ftiort radius of the 

 firft wheel afting upon the long radius of the fecond, which 

 was therefore with its crank at the flovvcft point of its move- 

 ment : but the decreafe of the motion, from the quickeft to 

 the floweft point of its revolution, being effefted by equal 

 increments, gave no fhock to the machinery. The crank 

 was of courie, during the flow half of its movement, pec- 

 forming its work ; and in the quick period, returning to 

 fetch its ftroke. By this judicious arrangement, the refift- 

 ance to the firft movement was very nearly equable : for 

 when it had work to perform, the wheel-work gained a 

 power upon the working point ; but ii,i returning, it caufed 

 it to urge the working point with fuch an increafed velocity, 

 as in fome degree counterbalanced the diminiftied refiftance : 

 but in this, no lofs was occafioned, becaufe this increafed 

 velocity ftiortened the period of inaftioB haftening the re^ 

 turn to a fituation for repeating its operation. 



Thefe elliptical wheels are, in the hands of an able me- 

 chanic, a very ufeful contrivance, but they have not been 

 much ufed in machinery, from the difBculties of forming 

 their teeth with precifion. In the Cometarium, (fee that 

 article,) they are introduced toreprefent the elliptic motions 

 of comets, and we have feen two inftances of their being 

 ufed in large mac hines, where they operated with as mucSi 

 facility as circular wheels. It is to be obferved, that a fmall 

 excentricity of the ellipfe, confequently a flight deviation 

 from the circular figure, will produce a great inequality of 

 their motion, becaufe the mcreafe of the afting radius of 

 one wheel, is attended with a correlpondent decreale of 

 the other, fo that to produce almoft any differences of mo- 

 tion which can be required in praftice, the excentricity of 

 the wheels will be luch as can eafily be accomphftied, and 

 as will work with each other fmoothly and accurately. 

 When heavy ftampers are to be raifed in order to drop on 

 the matter to be pounded, the wiper^, by which they are 

 lifted, fiiould be made 6f fuch a form that the ftamper 

 may be raifed by an uniform prefigure, or with a motio'n al- 

 moft imperceptible at firft. If this is not attended to, and the 

 wiper is only a pin fticking out from the axis, the ftamper 

 is forced into motion at once. This occafions a violent ' 

 jolt to the machine, and great ftrains on its moving parts 

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

 dually lifted at firft, the inequality of defultory motion is 

 never felt at the impelled point of the machine. 



We have feen pillons of pumps moved by means of a dou- 

 ble rack on the pifton rod : a half wheel takes hold of one 

 rack and raifes it to the required height. The moment 

 the half wheel has quitted that fide of tiie rack, it lays hold 

 of the other fide and forces the pifton down again. This 

 has been propofed as a great improvement, by correfting 



the 



