MECHANIC POWERS. <ffi 



pressure to overcome this repulsion, so as to make 

 bodies really touch each other. Sir Isaac Newton 

 states, that when two pieces of glass are placed 

 so as to appear to be in contact, they are still dis- 

 tant from each other T 5 5s<j of an inch ; and that 

 by pressure this distance is diminished, although 

 perhaps the contact is never quite perfect. 



Oil and water seem to repel each other; and 

 when drops of rain run over green cabbage leaves, 

 it may be seen that there is no actual contact, from 

 the light that is reflected from the under surface 

 of the drop, and from the leaf not being wetted. 

 Many other familiar instances might be adduced 

 of the existence of a repulsive power. Its laws are 

 less known than those of attraction, and its cause 

 is equally obscure. 



MECHANIC POWERS. 



The mechanical powers are simple instruments 

 by which men are enabled to raise weights, move 

 heavy bodies, and overcome resistances, which 

 they could not do with their natural strength alone. 



These simple machines are six in number: the 

 lever, the pulley, the wheel and axis, the inclined 

 plane, the wedge, and the screw; and of combi- 

 nations of these all mechanical engines consist. 



To calculate the effect produced by these, as 

 well as by all other machines, it is necessary, first, 

 to consider what must be the power to keep the 

 weight or resistance in a state of equilibrium or 

 balance : and for this purpose, in the theoretical 

 part of the calculation, all planes are considered 

 as perfectly smooth ; levers to be inflexible, and 

 without thickness or weight ; cords perfectly pli- 

 able, and without diameter ; and machines to have 

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