Scientific Lectures. 233 



three distinct meanings. In the first place, it is used to express the 

 cause of motion, as when we speak of the force of gunpowder; it is 

 also used to indicate motion itself, as when we refer to the force of a 

 moving cannon ball ; and, lastly, it is employed to express the eflfect 

 of motion, as when we speak of the blow which the moving bodjr 

 gives.* Because of this confusion, it has been found convenient to 

 adopt Rankine's suggestion,t and to substitute the word " energy " 

 therefor. And precisely as all force upon the earth's surface, using 

 the term force in widest sense, may be divided into attraction and 

 motion, so all energy is divided into potentfal and actual energy, 

 eynonymous with those terms. It is the chemical attraction of the 

 atoms, or their potential energy, which makes gunpowder so power- 

 ful ; it is the attraction or potential energy of gravitaticm which 

 gives the power to a raised weight. If, now, the impediments be 

 removed, the power just now latent becomes active, attraction is 

 converted into motion, potential into actual energy, and the desired 

 effect is accomplished. The energy of gunpowder or of a raised 

 weight is potential, is capable of acting ; that of exploding gunpow- 

 der or of a falling weight is actual energy or motion. By applying 

 a match to the gunpowder, by cutting the string which sustains the 

 weight, we convert potential into actual energy. By potential 

 energy, therefore, is meant attraction ; and by actual energy, motion. 

 It is in the latter sense that we shall use the word force in this lec- 

 ture ; and we shall speak of the forces of heat, light, electricity and 

 mechanical motion, and of the attractions of gravitation, cohesion, 

 chemism. 



From what has now been said, it is obvious that when we speak of 

 the forces of heat, light, electricity or motion, we mean simply the 

 different modes of motion called by these names. And when we saj 

 that they are correlated to each other, we mean simply that the mode 

 of motion called heat, light, electricity, is convertible into any of the 

 others, at pleasure. Correlation therefore implies convertibility, and 

 mutual dependence and relationship. 



Plaving now defined the use of the term force, and shown tliat 

 forces are correlated which are convertible and mutually dependent, 

 we go on to study the evidences of sucli correlation among the 

 motions of inorganic nature usually called physical forces; and to 



* lb., preface, p. vi. 



+ Rankine, W. J. M., Philosophical Magazine, Feb., 1853. Also Edinburgh Philosophical Journal, 

 July, 1855. 



