THE PROBLEMS OF GEOLOGY 529 



it is necessary to define the terms "force," "work," "energy," and 

 "agent." 



Hoskins defines "force" as action exerted by one body upon an- 

 other tending to change the state of motion of the body acted upon. 1 

 According to Daniell's more simple definition, "force" is any cause 

 of motion. 2 



When a force applied to a body moves the body in the direction 

 toward which the force acts, it does work. 8 In this sense "work" is 

 the product of force into displacement, the common formula being 

 W = FS. The unit of work is defined as the quantity of work done 

 by a unit force acting through a unit distance. 4 



Hoskins defines "energy" in the terms of force and work. Thus 

 he says when the condition of a body is such that it can do work 

 against a force or forces that may be applied to it the body is said to 

 possess energy. The unit of energy is the same as that of work. 5 

 According to DanielPs more simple definition /"energy" is the power 

 of doing work. 6 



The order of definition of the above terms is that in which know- 

 ledge of them has developed. The actions of forces in doing work are 

 observed. From such observations the existence of energy is inferred. 

 Wherever forces act upon matter and work is done, energy must exist. 

 Further reasoning shows us that bodies may possess energy which is 

 latent and is not exerting force. Hence many physicists have defined 

 "energy" without introducing the words "force" or "work." Thus, 

 according to Holman, "energy" is power to change the state of mo- 

 tion of a body. 7 If energy be recognized as the primary thing, then 

 "force" can be defined in terms of energy. According to Holman, 

 "force" is that action of energy by which it produces a tendency 

 to change the state of motion of bodies. 8 Similarly, the word 

 "energy" may be introduced into the definition of the word " work." 

 Thus Holman says "work" is that action of energy by which it pro- 

 duces motion in a free body, or produces or maintains the motion 

 of a body against resisting forces. 9 



An "agent" is any portion of the substance of the earth which may 

 exert force and thus expend energy to perform geological work. Thus 

 ether, air, water, and rock are agents. 



The next step in the comprehension of geological processes is a 

 consideration of the kinds of energies, forces, and agents, and their 

 relations. 



1 T. M. Hoskins, Theoretical Mechanics, pp. 2 and 16, 1900. 



" Alfred Daniell, A Text-book of the Principles of Physics, 3d ed. (1895), p. 4. 



Hoskins, op. cit., p. 298. 



Ibid., p. 298. 



Ibid., p. 308. 



Daniell, op. cit., p. 2. 



Silas W. Holman, Matter, Energy, Force and Work, p. 20, 1898. 

 8 Ibid., p. 41. Ibid., p. 17 



