Chemistry and Physics. 143 
and lightning. We can let the line ef represent the limit of 
resistance of the air if the field be drawn to scale; and we can 
thus trace the conditions that determine disruptive discharge. 
a a ee 
= = 
crn 
en ae ‘oe 
~——__- . at 
If the earth-surface be not flat but have a hill or a building, as 
H or L, upon it, then the lines of force and the equipotential 
lanes will be distorted, as shown in fig. 3. the hill or building 
so high as to make the distance H A or L / equal to e f (fig. 2), 
then we shall again have disruptive discharge. 
If instead of a hill or building we erect a solid rod of metal, GH, 
then the field will be distorted as shown in fig. 4. Now it is 
quite evident that whatever be the relative distance of the cloud 
= a eee, 
Z a 
“gat iG 
LASS 
BAM 
BaP 
se oat I Fe 
ag G eee 
and earth, or whatever be the motion of the cloud, there must be 
‘along which the lines of force must be longer than 
; and h 
Let us assume that a thunder-cloud is approaching the rod A B 
(fig. 5) from above, and that it has reached a point D’ where the 
distance D’ B is equal to the perpendicular height D’ C’. It is 
evident that, if the potential at D be increased until the striking- 
distance be attained, the line of discharge will be along D'‘C or 
