AT KEW OBSERVATOEY, 1898 TO 1912. 



141 



be claimed that we are getting an absolutely full measure of the potential gradient in 

 the open. The influence of the apparatus and observer it is believed is small, but it 

 may not be wholly infinitesimal. Then the site apart from the peculiarities already 

 mentioned does not altogether adequately represent an infinite plane. There are 

 now no fruit trees within 65 feet of the ground tube, but there are several low 

 buildings in the neighbourhood, one coming within about 70 feet of the site. Finally 

 the main Observatory building, the elevation of whose dome above the garden ground 

 is fully 60 feet, is only about 60 yards away, and there are four elm trees at distances 

 of from 55 to 80 yards, still of considerable height though they lost their tops many 

 years ago. There are sites less open to criticism in the Old Deer Park outside the 

 Observatory enclosure, but the time required to visit a distant site would be a serious 

 obstacle to its regular use, even if it were available. 



7. In computing the following tables all potential data for years prior to 1910 

 have been multiplied by 1'91. The spot to which all absolute values refer must be 

 regarded as the site of the apparatus for absolute observations, which is about 20 feet 

 above mean sea level in lat. 51 28' N, long. 19' W. 



Table I. gives the mean value of the potential gradient for each of the 180 months 

 of observation, with corresponding means for each year and each of the 12 months. 

 The results are from selected rainless days, free from negative potential, 10 in each 

 month, except in a few cases where 8 or 9 days only were available. 



TABLE I. Mean Monthly and Yearly Values of Potential Gradient 



(volts per metre). 



