44^ Apparatus for teaching Mechanics, 



in experiment III. Let a piece of web be put round the under radii, let one end of it be 

 nailed to the poll, and the other be held by the boy, and it will reprefent the application 

 of a rope to a moveable pulley ; if its motion be carefully confidered, it will appear that ' 

 the radii, as they fucceflively apply themfelves to the web, reprefent a feries of levers of 

 the fecond kind. A pulley is nothing more than an infinite number of fuch levers; the 

 cord at one end of the diameter ferving as a fulcrum for the organ during its progrefs. If 

 thxsjkeleton-pulley be ufed horizontally inftead of perpendicularly, the circumftances which 

 have been mentioned will appear more obvious. 



Upon the wooden road lay down a piece of girth-web j nail one end of it to the road ; 

 place the pulley upon the web at the other end of the board, and bringing the web over 

 the radii, let the boy, taking hold of it, draw the loaded fledge faftened to the hook at the 

 center of the pulley : he will draw nearly twice as much in this manner as he could with- 

 out the pulley*. 



Here the web lying on the road {hews more diftlnftly, that it is quiefcent where the 

 loweft radius touches it ; and if the radii, as they tread upon it, are obferved, their points 

 will appear at reft, whilil the center of the pulley will go as faft as the fledge, and the top 

 of each radius fucceffively (and the boy's hand which unfolds the web) will move twice as 

 faft as the center of the pulley and the fledge. 



If a perfon, holdings ftick in his hand, obferves the relative motions of the top, and 

 the middle, and the bottom of the ftick, whilft he inclines it, he will fee that the bottom 

 of the ftick has no motion on the ground, and that the middle has only half the motion of 

 the top. This property of the pulley has been dwelt upon, becaufe it elucidates the mo- 

 tion of a wheel rolling upon the ground ; and it explains a common paradox, which ap- 

 pears at firft inexplicable, " The bottom of a rolling wheel never moves upon the road." 

 This is aflerted only of a wheel moving over hard ground, which, in faft, may be confi- 

 dered rather as laying down its circumference upon the road, than as moving upon it. 



The inclined Plane and the Wedge. 



The inclined plane is to be next confidered. When a heavy body is to be ralfcd, it is 

 often convenient to lay a floping artificial road of planks, up which it may be puflied or 

 drawn. This mechanical power, however, is but of little fervice without the afllftance of 

 wheels or rollers : we (hall therefore fpeak of it as it is applied in another manner, under 

 the name of the wedge, which is in faft a moving inclined plane ; but if it is required to 

 explain the properties of the inclined plane by the Panorganon, the wooden road may be 

 raifed and fet to any inclination that is required, and the fledge may be drawn upon it as in 

 the former experiments. 



» In all thefe experiments with the flceleton-pulley fomebody muft keep it in its proper direftion ; as 

 from its ftrufture, which is contrived for iiluftration, not for prafUcal ufe, it cannot retain its proper fitua- 

 don without aiTiftance. 



Let 



