984 Marvels of the Universe 
LIGHTNING 
BY F. W. HENKEL, B.A., F.R.A.S., Author of ‘‘ Weather Science.” 
Tue developments of atmospheric electricity on a large scale resulting in the phenomena of a 
thunderstorm are amongst the most impressive of natural phenomena, and in many countries the 
wielding of the thunderbolt has been considered an especial attribute of the gods (Jupiter Tonans, 
etc.). The atmosphere is always in a condition of electric instability ; in fine weather generally 
positively charged, in stormy and rainy weather, often negative. It is probable that an electri- 
cally charged cloud is made up of a large number of charged drops of water which have derived 
their charges from evaporation. 
These drops condensing and 
uniting, the strength of their 
charges gradually increases, 
whilst at the same time the 
surface of the ground below 
becomes oppositely charged by 
the process known as induction, 
the layer of air between keep- 
ing the two opposite charges 
separate, serving, as it were, 
as a condenser. By degrees the 
difference of potential will be- 
come so great, and the inter- 
vening air will be so highly 
strained, that it will collapse, 
and a disruptive discharge take 
place, a spark or series of sparks 
will partly relieve the electric 
tension. Flash after flash then 
passes, till equilibrium is re- 
stored; the light being due to 
the intense heating causing 
the air to become luminous. 
Photo by] Tua This produces a sudden expan- 
THE SQUAT LOBSTER. sion (partial vacuum), followed 
A Crab whose shelly covering is rough with spiny scales. Like a real Lobster, by a rush of air from sur- 
it leaps rapidly backwards by the sudden flexure of the broad “’ tail.” " 
rounding regions, whence comes 
the sound, thunder. If the path of the spark be fairly straight and not very long, one sharp, 
loud clap is heard ; if it be long and broken, a succession of rolling and rattling sounds follows, 
with echoes from the clouds around. Since the speed of light is enormously great (one hundred 
and eighty-six thousand miles per second), the flash is at once seen, but the speed of sound being 
only about eleven hundred feet per second, or about one mile in five seconds, the reports from the 
various clouds at different distances will be heard in succession for some time after the lightning 
flash is seen. 
By counting the interval of time between the flash and thunder following, from the longer or 
shorter interval between these, the observer may ascertain whether the storm clouds are receding from 
or approaching towards his position, 
Three different kinds of lightning are commonly distinguished : (1) forked, or zigzag lightning ; 
