346 ACROPHYSICS. 



work entitled Isoporrika, concerning equiponderants. He also dis- 

 covered the important law of the equilibrium of fluids ; and applied 

 it to the finding of specific gravities, in the celebrated problem of 

 Hiero's crown. The invention of pumps for raising water, is due to 

 Ctesibius, and Hero, of Alexandria, 150 to 120 B. C. : and the first 

 correct ideas on the motion of water in canals, belong to Frontinus 

 of Rome, who flourished A. D. 100. The initial theory of Acous- 

 tics, or at least of musical sounds, belongs to Pythagoras ; and was 

 suggested, it is said, by the concordant notes of several hammers, 

 whose weights he found to have a certain ratio. 



The discovery of the parallelogram of forces, was made by Ste- 

 vens, or Stevinus of Holland, about A. D. 1600 ; to which Varignon 

 afterwards added the ratio of the sines of the angles. Galileo dis- 

 covered the laws of falling bodies, and invented the pendulum; 

 thus founding the branch of Dynamics. Torricelli's discovery of 

 the pressure of the air, and his invention of the barometer, were in 

 like manner the basis of Pneumatics, as already mentioned. Pascal 

 first noticed the principle of the transmission of pressure, afterwards 

 applied by Bramah to the hydrostatic press ; and Mariotte discovered 

 the law of pressure in gases when confined. Huygheris invented the 

 cydoidal pendulum, and explained its peculiar properties ; and con- 

 temporaneously with Wallis and Wren, he demonstrated the laws of 

 collision of bodies. Newton, in his Principia, or Principles of 

 Natural Philosophy, investigated the resistance of the air, and first 

 revealed the great law of Universal gravitation. Euler, by a happy 

 analysis, generalized the theorems of Mechanics, and reduced the 

 whole to a system of analytical formulas. James Bernouilli studied 

 the centre of percussion ; D'Alembert discovered the principle of 

 efficient and residual forces ; Coulomb investigated the laws of fric- 

 tion ; and Prony, those of running water : but many other discoveries, 

 in this branch of science, it is beyond our limits to notice. 



We proceed to explain some of the leading principles of me- 

 chanics, under the four heads of Statics, Dynamics, Hydrics, and 

 Pneumatics. 



1. The science of Statics, relates to the conditions of equili- 

 brium and of uniform motion, applied particularly to solid bodies. A 

 force is measured, by the velocity which it communicates to a given 

 mass : and the momentum, or quantity of motion, is equal to the 

 product of the mass into the velocity. The mass, is represented by 

 the weight; and is equal to the product of the bulk by the density; 

 which latter is the weight of the unit of mass. The velocity of a 

 body, is the space over which it moves in a unit of time ; as so many 

 feet per second. The resultant, of two or more forces, is a single 

 force, which might take the place of them all, and produce the same 

 effect. The forces which together are equivalent to the resultant, 

 are called components. A force equal and opposite to the resultant, 

 may be called a quiescent force ; as it produces equilibrium. 



If two forces act in the same straight line, their resultant is equal 

 to their sum, or difference, according as they act in the same, or in 

 opposite directions. If two component forces are oblique to each 

 other, but lie in the same plane, they will meet, and may be repre- 



