Apparatus for teaching Meckania. 443r 



advance, but if they pull fairly, there will be no advantage on cither part. In this ex- 

 periment the rope fhould pafs through the pulley P 3, and fhould be coiled round the 

 larger drum. And it muft be alfo obferved, that in all experiments upon the motion of 

 bodies, in which there is much fridtion, as where a fledge is employed, the refults arc 

 never fo uniform as in other circumftances. 



The Pulley. 



Upon the pulley we (hall fay little, as it is in every body's hands, and experiments may 

 be tried upon it without any particular apparatus. It fliould, however, be diftlndtly in- 

 culcated, that the power is not increafed by a fixed pulley. For this purpofe, a wheel 

 without a rim, or, to fpeak with more propriety, a number of fpokes fixed in a nave 

 fliould be employed, (Fig. 9.) Pieces like the heads of crutches fliould be fixed at the ends 

 of thefe fpokes, to receive a piece of girth- web, which is ufed inftead of a cord, becaufe a 

 cord would be unfteady ; and a ftrap of iron with a hook to it fliould play upon the center, 

 by which it may at times be fufpended, and from which at other times a weight may be 

 hung. 



Experime>a 9. Let this fkeleton of a pulley be hung by the iron ftrap from the tranfom 

 of the frame ; faften a piece of web to one of the radii, and another to the end of the 

 oppofite radius. If two boys of equal weight pull thefe pieces of girth-web, they will 

 balance each other ; or two equal weights hung to thefe webs will be in equilibrio. If a 

 piece of girth-web be put round the uppermoft radius, two equal weights hung at the ends 

 of it will remain immoveable ; but if either of them be pulled, or if a fmall additional 

 weight be added to either of them, it will defcend, and the web will apply itfelf fuccef- 

 fively to the afcending radii, and will detach itfelf from thofe that are defcending. If this 

 movement be carefully confidered, it will be perceived, that the web in unfolding itfelf,^ 

 a£l;s in the fame manner upon the radii as two ropes would if they hung to the extremities 

 of the oppoCte radii in fuccelTion, The two radii which are oppofite, may be confidered 

 as a lever of the firft fort, where the center is in the middle of the lever ; as each end 

 moves through an equal fpace, there is no mechanical advantage. But if this Ikeleton- 

 puUey be employed as a common Mock or tackle, its motions and properties will be entirely 

 different. 



Experiment lO. Fig. 9. Nail a piece of girth-web to a poft, at the dlftance of three or 

 four feet from the ground ; faften the other end of it to one of the radii. Faften another 

 piece of web to the oppofite radius, and let a boy hold the fkeleton-pulley fufpended by 

 the web j hook weights to the ftrap that hangs from the center. The end of the radius to 

 which the fixed girth-web is faftened will remain immoveable ; but, if the boy pulls the 

 web which he holds in his hand upwards, he will be able to lift nearly double the weight, 

 which he can raife from the ground by a fimple rope, without the machine, and he will 

 perceive that his hand moves through twice as great a fpace as the weight afcends : he has 

 therefore the mechanical advantage, which he would have by a lever of the fecond fort, as 



in 



