Relations of the Physical and Chemical Forces. 65 



electric machine, and the brake-wheel represent the 

 armature, we have precisely similar conditions for the 

 evolution of electric heat or of current electricity as exist 

 in the generation of heat by friction ; the dimensions of 

 the electro-magnet and the power of the magneto-electric 

 machine being strictly analogous to the length of the lever 

 and the weight acting upon it in determining the 

 magnitude of the result. 



I may also point out, in connexion with this illustration, , 

 the striking analogy which subsists between the cohesive 

 and magnetic forces in effecting the conversion of 

 mechanical power into heat and electricity, as well as the 

 important function which the cohesive force exercises in 

 its several modes in determining : (1) the essential 

 property of rigidity or stiffness of the lever, and (2) the 

 specific degrees of hardness of bodies through which, 

 under given conditions, the maximum of frictional heat is 

 generated. 



The various degrees of cohesive force have also their 

 analogues in the specific conductivities of bodies when 

 rotating in the magnetic field ; the highest conductivity 

 being correlated with the transformation of the greatest 

 amount of mechanical power into electric heat or current 

 electricity. The close agreement between the specific 

 conductivities of metals for heat and electricity, with 

 other analogies between these forces, are so well known 

 that I need only make a passing allusion to them. 



Although I have demonstrated the Archimedean 

 principle of the indefinite quantitativeness of mechanical 

 force by means of the lever, the principle might be well 

 illustrated by the inclined plane in its several forms, as 

 exemplified by the wedge and screw in solids ; by 

 hydraulic pressure in liquids ; and by pneumatic pressure 

 in gases through the wedge-like action of spherical 

 molecules forced tangentially against each other. 



E 



