332 SCIENCE PROGRESS 



bisecting that which joins their centres till it reaches the centre 

 of figure ; here, of course, no force is manifest, for the gravitation 

 effect of the heavy bodies is here equal and opposite. 



(2) The " polar axis " uniting the centres is another line of 

 force ; for a heavy particle lying on it will move along it to the 

 nearer pole ; or if at the centre will remain there. 



(3) Besides these crossed lines will be a set of curved lines 

 all convex to the equator and to the interpolar axis, but 

 concave to the prolongations of the interpolar axis. Such a 

 field has been termed a " crossed figure " or " antispindle." 



If, on the contrary, the force is a dual or bipolar force, 

 such as is represented by two conductors of opposite charges, 

 by the two isolated poles of a magnet (if it were possible to 



B 



Fig. 3. — Field between two centres of opposite sign, upper half of axial section. The 

 lines represent " unit lines." (From Watson's Text Book of Physics.) 



obtain such), by a source of heat and a refrigerator, by an 

 upwelling source and a sink, etc., then the distribution of the 

 lines of force or flow is different : 



(1) The interpolar axis and its prolongation still form one 

 such line. 



(2) The other lines of force run from pole to pole, as shown 

 in the figure : they form a series of curves, all concave to the 

 polar axis and convex to its ultrapolar prolongations. 



(3) The lines starting from the inner half of each pole, and 

 converging to it at either end, are what is called the interpolar 

 spindle; the others, of which only the slightly curved basal 

 parts are seen within the limits of the figure, constitute the 

 " circumpolar radiations " and " asters " ; but there is no marked 



