310 PETER GUTHRIE TAIT 



rods of iron. It often permanently magnetises steel, and in this way has been the 

 cause of the loss of many a good ship ; for the magnetism of the compass-needles 

 has been sometimes destroyed, sometimes reversed, sometimes so altered that the 

 compass pointed east and west. And by the magnetisation of their steel parts the 

 chronometers have had their rates seriously altered. Thus two of the sailor's most 

 important aids to navigation have been simultaneously rendered useless or, what is 

 worse, misleading ; and this, too, at a time when, because of clouds, astronomica 

 observations were generally impossible. All these dangers are now, however, easily 

 and all but completely avoidable. 



Sheet-lead punctured by lightning. 



A very singular effect of lightning sometimes observed is the piercing of a hole 

 in a conducting-plate of metal, such as the lead-covering of a roof. In such a case 

 it is invariably found that a good conductor well connected with the ground approaches 

 near to the metal sheet at the part perforated. We can easily repeat the experi- 

 ment on a small scale with tinfoil. [A thick piece of sheet-lead from the lower 

 buildings of Nelson's monument, Edinburgh, punctured by lightning, was exhibited. 

 It is figured in the woodcut, reprinted (by permission) from Proc, R.S.E., 1863.] 



The name thunderbolt, which is still in use, even by good writers, seems to have 

 been introduced in consequence of the singular effects produced when lightning 

 strikes a sandhill or sandy soil. It bores a hole often many feet in length, which 

 is found lined throughout with vitrified sand. The old notion was that an intensely 

 hot, solid mass, whose path was the flash of lightning, had buried itself out of sight, 

 melting the sand as it went down. It is quite possible that this notion may have 

 been strengthened by the occasional observation of the fall of aerolites, which are 

 sometimes found, in the holes they have made, still exceedingly hot. And at least 

 many of the cases in which lightning is said to have been seen in a perfectly clear 

 sky are to be explained in the same way. Everyone knows Horace's lines : 



