jfpparatus for teaching Mechanics. 445 



ends towards the other ; and can be faflened at any place by the fcrew of the focket. This 

 focket has two gudgeons, upon which it, and the lever which it contains can turn. This 

 focket and 'its gudgeons can be lifted out of the holes in which it plays, between the rail 

 R R, Plate XIX. Fig. 2. and may be put into other holes at R R, Fig. 5. Loop another 

 rope to the other end of this lever, and let the boy pull as before. Perhaps it fliould be 

 pointed out, that the boy muft walk in a direflion contrary to that in which he walked 

 before ; viz. from i towards 3. The height to which the weight afcends, and the dif- 

 tance to which the boy advances, fhould be carefully marked and meafured ; and it will be 

 found, that he can raife the weight to the fame height, advancing through the fame fpace 

 as in the former experiment. In this cafe, as both ends of the lever moved through equal 

 fpaces, the lever only changed the direcStion of the motion, and added no mechanical 

 power to the dire£t ftrength of the boy. 



Etcperiment 3. Shift the lever to its extremity in t\\e focket i the middle of the lever will 

 be now oppofite to the pulley, Fig. 4 ; hook to it the rope that goes through the pulley 

 P 3, and fallen to the other end of the lever the rope by which the boy is to pull. This 

 will be a lever of the fecond kind, as it is called in books of mechanics; in ufing which, the 

 reft/lance is placed betnueen the centre of motion or fulcrum, and the moving power. He will now 

 raife double the weight that he did in Experiment II. and he will advance through double 

 the fpace. 



Experiment 4. Shift the lever, and the focket which forms the axis, (without fliifting 

 the lever from the place in which it was in the focket in the laft experiment) to the holes 

 that are prepared for it at R R, Fig. 5. The free end of the lever E will now be oppofite 

 to the rope, and to the pulley (over which the rope comes from the fcale-beam). Hook 

 this rope to it, and hook the rope by which the boy pulls to the middle of the lever. The 

 eiFe£t will now be different from what it was in the two laft experiments ; the boy will 

 advance only half as far, and will raife only half as much weight as before. This is called 

 a lever of the third fort. The firft and fecond kinds of levers are ufed in quarrying ; and 

 the operations of many tools may be referred to them. The third kind of lever is em- 

 ployed but feldom, but its properties may be obferved with advantage whilft a long ladder 

 is raifed, as the man who raifes it is obliged to exert an increafing force till the ladder is 

 nearly perpendicular. When this lever is ufed, it is obvious, from what has been faid, that 

 the power muft always pafs through lefs fpace than the thing which is to be moved ; it can 

 never, therefore, be of fervice in gaining power.^ But the objeft of fome machines is to 

 increafe velocity, inftead of obtaining power, .as in a fledge-hammer moved by mill- work.. 

 (V. the plates in Emerfon's Mechanics, No. 236.) 



The experiments upon levers may be varied at pleafure, increafing or diminifhing 

 the mechanical advantage, fo as to balance the power and the refiftance, to accuftom 

 the learners to calculate the relation between the power and the effeft in different 

 circumftances i always pointing out, that whatever excefs there is in the power*, or 



* The word potver is here ufed in a popular fenfe, to denote the ftrength or effiacacy that is employed to 

 produce an efFeft by means of any engine. 



in 



