199 
1912-13.] Records of Atmospheric Electric Potential. 
corresponding points. Kahler * found a mean lag of 3*9 minutes, and 
showed that this lag depended on cloud-drift, and to a lesser degree on 
wind direction and strength. 
A difference of 4 minutes between the two stations might be due to 
errors in measuring the curves, and in the few cases where there seems 
to be evidence for a lag this is probably due to the two points measured 
not being exact points of correspondence. 
There is thus no conclusive evidence for a lag due to wind or cloud 
drift. More accurate means of measuring time, and also fuller data with 
regard to cloud drift, would be necessary for a proper investigation of the 
question. The prevailing wind in Edinburgh being westerly, and the two 
stations being north and south of one another, it is not to be expected 
that the mean lag would be a very large one. Assuming a mean velocity 
of wind for these disturbed days at 20 miles per hour, and the distance 
between the stations as 1*8 miles, then with a south wind we would expect 
a lag of the Laboratory curve behind the Observatory curve of about 
5 minutes, while with any other wind direction the lag would be smaller. 
The differences of magnitude of the variations which are evident 
between the two stations are probably due to the difference between the 
purity of the air, any change of conductivity in the air being accompanied 
by a greater effect at the Blackford Hill than at the Laboratory, on 
account of the very much larger number of soot and dust particles present 
in the neighbourhood of the latter station. These act as nuclei to which 
the ions get attached, their velocities being thus considerably reduced. 
There is one other difference between the stations which may have some 
effect in giving rise to differences in the curves. The water-droppers at 
the two stations differ in height above the ground. It has been shown that 
in Paris f considerable differences are obtained in the curves showing vari- 
ation of potential at different heights above the ground. Any change 
which was merely a change in the layers of the air nearest the ground 
might affect one station and leave the other unaffected. 
Generally speaking, however, the influence of the town air is much 
smaller than might have been expected, and the curves obtained in the 
town and those at a station on the outskirts of the town distant 1*8 miles 
are more or less similar, the difference being, generally speaking, difference 
of magnitude. 
* Kahler, loc. cit. t Chauveau, V Eleclricite atmospherique, Paris, 1902. 
{Issued separately May 19, 1913.) 
