PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 639 



of a molten mass, the question naturally arises, by what is it pro- 

 tlucecl ? I answer, gravity. 



The idea that force must produce motion, is a mistaken one. 

 Force is what produces or tends to produce motion. Hence it 

 exists in two states or conditions, one of action, and the other of 

 tension ; when in the former it it is dynamical, in the latter, kinetic 

 or statical. 



As a force is only dynamical when transferring from one body 

 to another the means by which the transfer is made, being motion, 

 the dynamical is merely a temporary condition of force, the 

 kinetic, permanent. 



As I hold this book in my hand, it is acted on by a force which 

 tends to draw it towa>"ds the earth's centre of gi-avity. I let it 

 fall ; while falling, the force of gravity is dynamical ; while at 

 rest, it is kinetic. 



Again, all forces can be divided into two great classes; forces of 

 attraction and of repulsion, the sum of each being equal. Any 

 force can be converted into any other of the same kind, but not 

 into one of the other. Gravity is a force of attraction ; heat, of 

 repulsion. One cannot exist v/ithout the other ; they are, like all 

 other forces, necessary attributes of matter, and are also correla- 

 tive and antithetical to each other. When gravity is dynamical, 

 or in other words, is transferring itself from one body to another, 

 an equal amount of heat is set free. When gravity is kinetic, an 

 equal amount of heat is also kinetic. 



Thus the force of attraction acting on this book may be repre- 

 sented hyaline drawn from it to the earth's centre of gravity; the 

 amount of heat also acting on it must likewise be represented by 

 the same line, both at present being kinetic. I drop the book, 

 and a certain portion of the gravity, represented by the distance 

 through which it falls, is set free or becomes dynamical, and so 

 does an equal amount of heat. 



To answer the questions, " AVhat has ))ecomo of the gravity set 

 free?" and " since the force of gravity varies inversely as the 

 s»quare of the distance, is it not greater after than before the book 

 fell?" an explanation of a theoiy advanced by Helraholtz, and 

 indorsed by Faraday, is ncccssar}-. "The intensity of the force of 

 gravity varies inversely as the sqiiarc of the distance, but not the 

 mnount of force. A force is measured by the distance it can move 

 a given body." Before this book vfas dropped, the force of 

 gravity tended to move it through u greater distance than now; 



