400 JAMES CLERK MAXWELL. [CHAP. XII. 



electric force normal to the surface and just outside it. 

 This causes the surface to rise wherever the normal force is 

 great, or close to the electrodes. 



The science of displacements is in Euc. I. 4, etc., and 

 wherever one figure is placed upon another. It belongs to 

 the method of contemplating the relations of two figures 

 which may be supposed to co-exist, though we may also 

 suppose that they are copies of the same figure in different 

 positions. 



But just as we assume that distance is a continuous 

 quantity capable of measurement, though all our attempts at 

 measurement are made with instruments made of non-rigid 

 and discontinuous matter, so we may assume that time is 

 a continuously flowing quantity capable of measurement, 

 though we have not yet found out any accurate method of 

 comparing distant intervals of time. 



Now Kinematics requires no more than this notion of 

 time, as the common independent variable t. If we 

 suppose that r is that (unknown) which flows uniformly, 

 then for kinematical purposes it is enough that t is a func- 

 tion of T ; but when we come to Kinetics proper we must have 



cPt 



2 very small. 



GUT 



Have you read Julius in Nature, about the beginning of 

 June ? [14th June]. 



The most constant things we know are the properties of 

 bodies. For instance, water in equilibrium with ice and 

 vapour gives us a good deal. 



I. A unit of density (not the orthodox one) = D. 



L 



II. A unit of pressure (too small for practical use) 



^ = P 

 LT 2 



III. A unit of time (namely, the time of revolution of 

 a satellite just grazing a sphere of water) = T. 



These three quantities being independent of each other 

 give M, L, and T. 



