T. Deighton 213 



but R, + F, = B,,. 



^ab + "ac ~ ^ic 



Similarly R^ and R,. may be determined and R,i will equal i?^,, — R^- 

 Obviously R^^ and R^^ can then be calculated. As will be seen no 

 very good agreement is attained. 



Table IV. Resistances, observed and calculated, on 6" square. 



This method of calculation gave results more in accordance with 

 the observations when seven electrodes were set out in the angular 

 points of a regular hexagon with the seventh in the middle, in such a 

 manner that the distance between any adjacent pair was the same = 18" 

 corresponding to (Gardner's limit: the value of an electrode being de- 

 termined as the mean of the results obtained from two or three triangles. 

 As these results are very length v and of no importance in themselves 

 they are omitted. The calculated and observed results agreed to about 

 100 ohms in 3000. It will be seen later that in an infinite isotropic 

 medium the calculation should give the result desired, the variations 

 are therefore most probably due to local aelotropic conditions. 



Resistance at greater Distances. The question of what happens at 

 distances much exceeding 18" was investigated by the author, once as 

 a continuation of the 7" depth observations of Table II and once inde- 

 pendently of these. The results, given in Table V, show a slow increase 

 of resistance beyond 18" up to quite long distances. We are therefore 

 driven to conclude that had Gardner actually measured the resistance 

 between two of the smaller electrodes at 15' instead of merely calculat- 

 ing it he would not have concluded that the intervening ISi' of soil 

 accounted for only 2 per cent, of the resistance at 15', the rest being 

 due to the 9" of soil about the electrodes. 



It will be observed that in Exp. 2 there is no minimum at 18". There 

 is here no real discrepancy, the occurrence of a minimum being a possible 



