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LECTURE XII. 



ON STATICS. 



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The examination of the magnitude of the various forces, employed in prac- 

 tical mechanics, constitutes the doctrine of statics. The term statics, in a 

 strict sense, implies the determination of weights only; but it may without 

 impropriety be extended to the estimation of forces of all kinds, especially 

 active forces, that can be compared with weights, in the same manner as the 

 term hydrostatics comprehends every thing that relates to the equilibrium of 

 fluids. The measurement of the passive strength of the materials employed, 

 the changes produced in them by the forces which they resist, and the laws 

 of the negative force of friction, are also subjects immediately introductory to 

 the particular constructions and uses of machinery, and nearly coimected 

 with the department of statics. 



The art of weighing is peculiarly important, as it furnishes us with the 

 only practical mode of determining the quantity of matter in a given body. 

 We might indeed cause two bodies to meet each other with known velocities, 

 and from the effects of their collision, we might determine their comparative 

 momenta, and the proportion of their masses; but it is obvious that this pro- 

 cess would be exceedingly troublesome, and incapable of great accuracy; we 

 therefore recur to the well known law of gravitation, that the weight of 

 every body is proportional to the quantity of matter that it contains, and we 

 judge of its mass from its weight. If all bodies were of equal density, we 

 might determine their masses from their external dimensions ; but we seldom 

 find even a single body which is of uniform density throughout; and even if 

 we had such a body, it would in general be much easier to weigh it correctly 

 than to measure it. 



The weight of a body is commonly ascertained, by comparing it immediately 



