THE DIVIDING CELL 



331 



"lines of force." His conception was extended and developed 

 by Clark-Maxwell, who writes : " If we draw a line such that at 

 every part of its course it is coincident in direction with the 

 force at that point, this line may be called a ' line of force,' 

 since it indicates the direction of the force at every part of 

 its course." : It will be noted that such a line is a geometrical 

 line, not a material line. Maxwell also recognised that for a 

 field of two " unlike " poles the lines have the distribution of 

 stream-lines in liquid in relation to an upwelling source and 

 an engulfing sink, and, like Kelvin, saw that this distribution 

 was identical with the lines of the flow of heat in a conductor 



Fig. 2. — Diagram of the crossed field or antispindle, showing the directions of lines of force 

 between two like poles. The lines shown are not "unit lines." 



between a source of heat and a refrigerator ; while the field 

 of two " like " poles corresponded with the thermal field of two 

 sources of heat, or two refrigerators, as the case might be. 



Now, if we consider the force of gravity, which is, as we 

 have seen, a simple force of uniform sign, and draw the diagram 

 for two equal "centres" or gravitating spheres, we shall have 

 this arrangement (fig. 2) : 



(1) Any heavy particle on a plane equidistant from the two 

 centres — the "equatorial plane" — will move in a straight line, 



' In fig. 2 we have approximately equidistant lines of force, but they do 

 not represent the distribution of "unit lines" which we define below (p. 334). 

 If these were represented it would be seen that the area between the centres is 

 the weakest part of the field, and that midway the force sinks to zero. 



