156 THUNDERBOLTS 



peak (■w'licre an enterprising journalist not long ago dis- 

 covered the remains of Noah's Ark), has been riddled 

 through and through by frequent lightnings, till the rock 

 is now a mere honeycombed mass of drills and tubes, like 

 an old target at the end of a long day's constant rifle 

 practice. Pieces of the red trachyte from the summit, a 

 foot long, have been brought to Europe, perforated all over 

 with these natural bullet marks, each of them lined with 

 black glass, due to the fusion of the rock by the passage of 

 the spark. Specimens of such thunder-drilled rock may 

 be seen in most geological museums. On some which 

 Humboldt collected from a peak in Mexico, the fused slag 

 from the wall of the tube has overflowed on to the sur- 

 rounding surface, thus conclusively proving (if proof were 

 necessary) that the holes are due to melting heat alone, 

 and not to the passage of any solid thunderbolt. 



But it was the introduction and general employment of 

 lightning-rods that dealt a final deathblow to the thunder- 

 bolt theory. A lightning-conductor consists essentially of a 

 long piece of metal, pointed at the end whose business it 

 is, not so much (as most people imagine) to carry off the 

 flash of lightning harmlessly, should it happen to strike the 

 house to which the conductor is attached, but rather to pre- 

 vent the occurrence of a flash at all, by gradually and 

 gently drawing off the electricity as fast as it gathers before 

 it has had time to collect in suflicient force for a destructive 

 discharge. It resembles in eft'ect an overflow pipe which 

 drains off' the surplus water of a pond as soon as it runs 

 in, in such a manner as to prevent the possibility of an 

 inundation, which might occur if the water were allowed 

 to collect in force behind a dam or embankment. It is a 

 flood-gate, not a moat : it carries away the electricity of the 

 air quietly to the ground, without allowing it to gather in 

 Bufficient amount to produce a flash of lightning. It mighc 



