140 DE. C. CHKEE: ATMOSPHERIC ELECTRICITY POTENTIAL GRADIENT 



This was less easy than might appear at first sight. The ground immediately round 

 .the old stand was gravel, and slightly different in level from the surrounding grass. 

 The observations with the old apparatus referred to a point directly over the stand, 

 which was a fixture. There was thus no possibility of taking observations with the 

 old and the new apparatus at exactly the same spot. The observations, moreover, 

 were not so numerous as might have been desired. The mean result made the 

 potential gradient deduced by the old and the new apparatus stand to one another 

 in the ratio I'OO to 1'65. 



When the comparison was made there were several fruit trees near the grass plot, 

 and the observations made with the new apparatus suggested the expediency of their 

 removal. It was accordingly decided to cut down the trees at the year's end, and to 

 start the new year, 1910, with the new apparatus. Unfortunately circumstances 

 did not allow of any direct determination of the effect of removing the trees. Thus 

 Mr. DIXES' observations did not suffice for the determination of a factor to be applied 

 to results obtained prior to 1910 to bring them to what they would have been under 

 the conditions since prevailing. This, however, can, I think, be done fairly 

 satisfactorily by considering the values obtained for the mean annual potential 

 gradient. These have been as follows : 



Old apparatus. 



New apparatus. 



Year. . . 1898. 1899. 1900. 1901. 1902. 1903. 1904. 1905. 1906. 1907. 1908. 1909. 1910. 1911. 1912 



P.O. ... 161 179 141 156 145 162 167 167 156 163 148 164 



310 301 



300 



If we divide the 12 years during which the old apparatus was in use into the period 

 1898 to 1904, treated in E 1} and the later period 1905 to 1909, we find 1587 as the 

 mean potential for the former period, and 159'6 as the mean potential for the latter. 

 There is thus no indication of any progressive change in the value of the potential 

 gradient. Moreover, the mean derived from any three successive years departs but 

 little from the mean 159'1 derived from the whole 12. We are thus unlikely to be 

 much in error if we regard the means 159'1 and 3037, derived respectively from the 

 years when the old and the new apparatus was in use, as representing the same real 

 potential gradient. This gives 1'91 as the factor to be applied to results obtained 

 prior to 1910 to bring them up to what they would have been if the new apparatus 

 had been in use under the conditions now existing. 



When preparing E t I did not suspect that so large an underestimate was being 

 made of the absolute value of the potential gradient, but I quite realised that a sensible 

 correction was probably necessary, and for that reason, amongst others, a number of 

 the results were expressed as ratios, not as absolute potentials. Even now it cannot 



