216 
ME. C. V. WALKEE ON MAGNETIC CALMS AND EAETH-CIJEEENTS. 
over two months, to satisfy myself that the one section is under all circumstances less 
active in derived currents than the other. This difference can only be attributed, as 
already suggested in my former communication (p. 109), to the different geological con- 
ditions of the two sections of country, a difference which may operate in two ways: 
either the resistance of the section may be relatively great, so that the earth-plates 
penetrate into a portion of the electric plane that is traversed by a ciuTent of low value, 
and hence the derived current is comparatively low; or the resistance of the section 
may be relatively small, so that, although the earth-plate may penetrate into a portion 
of the electric plane that is traversed by a current of higher value, yet the wire resist- 
ance, in contrast with the high conducting power of the earth section, may cause the 
derived current to have a relatively low value. 
The London-Hastings line makes an angle of possibly 70° with the N.E. or S.W. 
resultant, that is with the m, d, u or the d^ u, d currents ; so that even with a good 
geological section and with a perfect knowledge of the relative resistances of its various 
parts, it would be no easy matter even to hint at the precise relation between the value 
of the current and the structure and arrangement of the strata. Mr. Robert Hunt has 
kindly furnished me with a geological section of the country between London and 
Hastings drawn by Mr. F. Drew. Between London and Tonbridge are included, London 
clay; Woolwich beds (sand); chalk; gault (clay); lower greensand, sand and a httle 
limestone ; clay ; sand and sandstone ; clay. Between Tonbridge and Hastings, sand 
and sandstone ; clay and a little limestone. There are many faults also between Ton- 
bridge and Hastings. 
Column 1 of Table XVII. contains the values given by the whole length, from London 
to Hastings. They differ but little, save in one or two instances, from the Tonbridge- 
Hastings values ; and are consequently very low in comparison with the Tonbridge- 
London values. These facts all indicate the very notable influence of local conditions, 
other than the meteorological variation already noted, over the relative value of the 
current in different parts of the plane. 
It was a matter of considerable importance to determine with certainty whether the 
currents with which I was dealing were in whole, or only in part, earth-currents; 
whether, that is, any portion of the observed deflections of the galvanometer needle 
were due to electricity collected from the atmosphere by the suspended telegraph wires 
that were employed in these observations. To determine this I availed myself of the 
assistance of Ashford. After taking observations in the usual way between London and 
Hover, and between Tonbridge and Dover, I desmed the Ashford clerk to detach the 
wme from his instrument and leave the end insulated. I thus had a wire of 67 miles 
connected with the earth at London, or one of 25 miles connected with the earth at 
Tonbridge, the end in either case being insulated at Ashford. It was desirable to make 
a considerable number of observations, at various hours of the day and under all 
conditions of weather, in order to test this question rigidly. The results are given in 
Table XVHI. 
