60 



FERGUSON'S LECTURES. 



LECT. 

 III. 



power, it is never used but in cases of necessity ; such 



, as that of a ladder, which being fixed at one end, is by 



the strength of a man's arms reared against a wall. 



And in clock-work, where all the wheels may be 



reckoned levers of this kind, because the power that 



moves every wheel, except the first, acts upon it near 



the center of motion by means of a small pinion, and 



the resistance it has to overcome, acts against the teeth 



round its circumference. 



The fourth The fourth kind of lever differs nothing from the 

 lever, first, but in being bended for the sake of convenience. 



A C B is a lever of this 



sort, bended at C, which 



is its prop, or center of 



motion. P is a power act- 

 ing upon the longer arm 



A C at F, by means of the 



cord D E going over the 



pulley G ; and Wisa. weight 



or resistance acting upon 



the end B of the shorter 



arm B C. If the power be 



to the weight, as C B is 



to C F, they are in equili 



brio. Thus, suppose W to 



be five pounds acting at 



the distance of one foot 



from the center of motion 



C, and P to be one pound 



acting at F, five feet from 



the center C, the power and 



weight will just balance 



each other. A hammer 



drawing a nail is a lever of this sort. 



The wheel 2. The second mechanical power is the wheel and axle, 

 ' in which the power is applied to the circumference of 



the wheel, and the weight is raised by a rope which coils 





 p 



