156 LECTURE XVIII. 



LECT. XVII. ADDITIONAL AUTHORITIES. 



Cumming's Elements of Clock and Watch Work, 4 to, Lond. 1766. Lepaute, 

 Traite d'Horlogerie, 4to, Par. 1767. Berthoud's Works, viz. Essai sur 1'Horlo- 

 gerie, 2 vols. 4to, Paris, 1763. Traite des Horloges Marines, 4to, 1773. Surl'In- 

 vention, &c. des Machines proposees en France pour la Determination des Longi- 

 tudes par la Mesure du Temps, 4to, 1773. Supplement, 1787. Trait6 des Mon- 

 tres a Longitudes, 4to, 1792. Suite, 1797. Supplement, 1807. Robison, Mech. 

 Phil. Reid, Treatise on Clock and Watch Work, Edin. 1826. Prony, Note sur un 

 Nouveau Moyen de regler la Duree des Oscillations des Pendules, 4to, Paris. 

 Jurgensen, MSmoires sur 1'Horlogerie Exacte, 4to, Paris, 1832. 



LECTURE XVIII. 



ON RAISING AND REMOVING WEIGHTS. 



THE methodical arrangement of our subject leads us, after having con- 

 sidered the modifications of force, to those machines which are intended for 

 counteracting it, or for producing motion in opposition to an existing force. 

 The simplest of the forces to be counteracted is gravitation, and it is one 

 of the most common employments of mechanical powers to raise a weight 

 from a lower to a higher situation. This operation is also intimately 

 connected with the modes of overcoming the corpuscular force of friction or 

 adhesion, which constitutes the principal difficulty in removing bodies 

 horizontally from place to place ; for if we had only to produce motion in 

 an unresisting mass of matter, a loaded waggon might in time be drawn 

 along by a silk worm's thread. The raising and removing of weights, 

 therefore, together with the modes of avoiding friction in general, constitute 

 the first part of the subject of the counteraction of forces, and the remain- 

 ing part relates to the machinery intended for overcoming the other cor- 

 puscular powers of bodies by such operations as are calculated to change 

 their external forms. 



Machines for raising weights, which involve only the mechanics of solid 

 bodies, are principally levers, capstans, wheels, pullies, inclined planes, 

 screws, and their various combinations in the form of cranes. 



A lever is a very simple instrument, but of most extensive utility in 

 raising weights to a small height. We may recollect that levers are distin- 

 guished into two principal kinds, accordingly as the power and weight 

 are on different sides or on the same side of the fulcrum ; the forces 

 counteracting each other being in the one case in the same direction, in the 

 other, in opposite directions. Thus, when a man lifts a stone by means of 

 a lever of the first kind, resting on a fulcrum between himself and the 

 stone, he presses down the end of the lever, and the utmost force that fre 

 can apply is equal to the whole weight of his body ; but when he thrusts 

 the lever under the stone, so that its extremity bears on the ground, it 

 becomes a lever of the second kind, and in order to raise the stone, he must 



