MECHANICS. 345 



of them, in the brief manner here required, under the branches of 

 1. Mechanics; 2. Astronomy; 3. Optics; 4. Ceraunics, including 

 Calorics, Electricity, Magnetism, and Galvanism ; and 5. Chemistry. 



CHAPTER I. 



MECHANICS. 



MECHANICS, is the science which treats of forces acting upon 

 matter, and which investigates the laws of equilibrium and motion, 

 both of solids, and of fluids. The name is derived from the Greek 

 M'?*"^, a machine; as the construction of machines probably first 

 led to the study of this science. The term matter, has been already 

 defined, and the principal properties of matter explained, in the intro- 

 duction to this department. A force, is an agent, tending either to 

 produce or to resist motion. When the forces acting upon a body 

 counteract each other, or do not produce any motion, the body is 

 said to be in equilibrium. When a body moves through equal 

 spaces in equal times, its motion is said to be uniform ; but in all 

 other cases it is variable. In the former case, the forces cease to 

 act, or else counteract each other : but all cases of variable motion 

 are owing to the action of continuous or incessant forces. 



The best sub-division of Mechanics, is probably into the four heads 

 of Statics, Dynamics, Hydrics, and Pneumatics. Statics, treats of 

 the conditions of equilibrium, and of uniform motion, particularly in 

 regard to solids ; though many of its principles are also applicable to 

 fluids. Dynamics, treats of the laws of variable motion ; with the 

 same restriction concerning its application. Hydrics, including both 

 Hydrostatics and Hydrodynamics, treats of those laws of equilibrium 

 and motion which are peculiar to liquids ; and Pneumatics, treats 

 of the corresponding laws, in so far as they are peculiar to aeriform 

 fluids, or gases. The term Hydraulics, more properly applies to 

 those constructions for the conveyance of water, the study of which 

 belongs to Civil Engineering ; and Acoustics, or the doctrine of 

 sound, may properly be included under the head of Pneumatics. 

 The science of Mechanics, finds its applications not only in the 

 construction of Machinery, but also in the succeeding branches of 

 the present department ; to some of which, the study of it is an 

 indispensable preliminary. 



According to Vitruvius, the ancients were from time immemorial 

 acquainted with several of the mechanical powers, so called ; as the 

 inclined plane, capstan, and pulley ; to which, no doubt, should be 

 added the wedge, and the lever, as the simplest of them all. The 

 screw was also known to, if not invented by, Archimedes ; to whom 

 the theory of the mechanical powers is justly attributed. The most 

 ancient writings extant, on this science, are those of Aristotle ; who 

 understood the principle of momenta, but not that of the lever. This 

 latter principle was first discovered by Archimedes ; who deduced 

 from it the principle of the centre of gravity, as explained in his 

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