384 WRITINGS OF JOSEPH HENRY. [1855- 



During very warm weather a feeble current is observed at 

 different periods of the day, which may be referred to thermo- 

 electricity. It is well known that when one end of a long 

 conductor is heated and the other cooled, a current of elec- 

 tricity will pass from the hotter to the colder extremity, and 

 this will be continued as long as the difference of tempera- 

 ture exists. Extended lines in a north and south direction 

 are most favorably situated for observing a current of this 

 class. Currents of electricity of sufficient intensity to set 

 fire to pieces of paper, have also been observed in connection 

 with the appearance of the aurora borealis 



Means of Protecting Buildings. 



Although much has been written and said in disparage- 

 ment of the admirable invention of our illustrious country- 

 man, Franklin, yet an attentive consideration of all the 

 facts, even independent of theory, fully establishes its great 

 importance. 



1st. It is well known, from general experience, that light- 

 ning directs itself to the most elevated portions of edifices. 

 Cotton Mather declares that lightning is under the immedi- 

 ate direction of the " Prince of the powers of the air," because 

 church steeples are more frequently struck than any other 

 objects. It is therefore evident that the preservative means, 

 whatever they may be, should be applied to the upper por- 

 tions of a building. 



2d. If other conditions be the same, lightning directs itself 

 in preference — to metals. When therefore a mass of metal 

 occupies the more elevated portion of a house we may be 

 nearly certain that lightning, if it falls upon the building, 

 will strike that point. 



3d. Lightning when it enters a metallic mass does mis- 

 chief only where it quits the metal, and in the vicinity of 

 the point at which it issues. A house therefore entirely 

 covered with metal would be safe, provided this covering 

 were intimately connected with the ground by metallic con- 

 ductors of sufficient size. When there are upon the roof 

 or in any of the upper stories of an edifice several dis- 



