522 PHYSICAL GEOGRAPHY. 



lightning a fate which Professor Richman of St. Petersburg experienced, while per 

 forming incautiously the sublime experiment of Franklin, measuring the strength of the 

 electricity brought down by a metallic rod in a thunder-storm, being instantly killed. 

 Pliny likewise mentions the laurel as the only earthly production which lightning does 

 not strike ; hence, as a protection, these trees were planted around the temple of Apollo. 

 Columella, however, mentions white vines surrounding the house of Tarchon, the 

 Etruscan, for the same purpose. These expedients may provoke a smile without deserving 

 one ; for there can be no doubt that trees sufficiently high around a temple, or succulent 

 plants covering a dwelling, will exercise to some extent a protective power, and act as a 

 regular system of conductors. Salverte mentions several medals which appear to have 

 reference to this subject, particularly one which represents the temple of Juno, the 

 goddess of the air, the roof of which is armed with pointed rods. He quotes also Michaelis, 

 upon the temple of Jerusalem, to show that the Jews were not unacquainted with the 

 art of protecting their public buildings a position grounded upon the following facts : 

 " 1. That there is nothing to indicate that the lightning ever struck the temple of Jeru 

 salem during the lapse of a thousand years." This, of course, does not make the fact 

 certain; but when, as M. Arago justly remarks, we consider how carefully the ancient 

 authors recorded the cases in which their public buildings were injured by lightning, we 

 may accept the silence observed respecting the temple of Jerusalem, as proof that it was 

 never struck. For three centuries the cathedral of Geneva, the most elevated in the 

 city, has enjoyed a similar immunity, although inferior buildings have been repeatedly 

 damaged. Saussure discovered the reason of this, in the tower being entirely covered 

 with tinned iron plates, connected with different masses of metal on the roof, and again 

 communicating with the ground by means of metallic pipes. " 2. That according to the 

 account of Josephus, a forest of spikes with golden or gilt points, and very sharp, covered 

 the roof of this temple ; a remarkable feature of resemblance with the temple of Juno 

 represented on the Roman medals. 3. That this roof communicated with the caverns in 

 the hill of the temple, by means of metallic tubes, placed in connection with the thick 

 gilding that covered the whole exterior of the building ; the points of the spikes there neces 

 sarily producing the effect of lightning rods. How are we to suppose that it was only by 

 chance they discharged so important a function ; that the advantage received from it had 

 not been calculated ; that the spikes were erected in such great numbers only to prevent 

 the birds from lodging upon and defiling the roof of the temple ? Yet this is the sole 

 utility which the historian Josephus attributes to them." Curious as are these facts, they 

 will by no means justify the conclusion that the ancient world had any acquaintance with 

 the art of guiding the electric fluid from the bosom of the clouds, conducting it in a 

 prescribed course, and thus disarming it of its terrors. 



The subject of electrical agency is intimately connected with that of magnetism, to 



which this is the fittest place to glance, one of the most recondite points of physical 



science. The two forces were long supposed to be related, from the fact that lightning 



often renders steel magnetic, and disturbs the magnetic needle. But they are now known 



to be identical, or rather, different forms under which the same power manifests itself. 



The magnetic agency, like electricity, has a general distribution over the earth, but the 



phenomena differ in different parts of the world, and are subject to periodical differences 



in the same place, the cause of which is very little understood. Every one is acquainted 



with the polarity of a freely suspended magnetic needle, or its tendency to lie parallel 



with the earth's axis, pointing nearly north and south in every region of the globe. What 



is called the dip or inclination of the needle is its divergence from a perfectly horizontal 



position. Thus the north pole of the needle inclines downwards in the latitude of London 



at an angle of 70, but conveyed towards the equator, the dip diminishes, till no inclina- 



