686 THE POPULAR SCIENCE MONTHLY, 



alike to ascertain what it is that lightning spares, as "well as what it 

 strikes. It is of the utmost importance, for the advance of man's 

 knowledge in this branch of physical investigation, that all instances 

 of injury from lightning should be immediately examined and tested, 

 and that all facts ascertained should be accurately described and placed 

 upon record. 



In the year 1866 the Minister of War in France became doubtful 

 in regard to the measures which were then taken to secure powder- 

 magazines against accident from lightning, and in consequence once 

 again brought the matter formally under the consideration of the 

 Academy of Sciences. It was this action of the minister which led 

 to the third report, also drawn up by M. Pouillet, adopted by the 

 Academy in the beginning of 1867, and shortly afterward issued un- 

 der the authority of the French Government. In this report the best 

 method of making joints in a conductor by overlapping, riveting, and 

 soldering the contiguous ends, was pointed out, and it was urged that 

 the underground continuation of the rod should be carried on to an 

 adequately moist place, even if miles had to be traversed for the pur- 

 pose. The increase in the number of air-terminals and the connecting 

 them together were deemed of more consequence than the increasing 

 the height of a smaller number. Secondary terminals were advised 

 for every additional length of thirty-three yards of roof. The expan- 

 sion of rods by heat was provided for by inserting free semicircular 

 bands of red copper at suitable intervals, four inches of addition to 

 the length being allowed for in every hundred yards of rod. 



The example set by France in the preparation of these reports was 

 followed for the first time in England by the appointment of a Naval 

 Commission in 1839 to inquire into the protection of the vessels of the 

 Royal Navy. This commission was formed in consequence of the pub- 

 lic attention which had been drawn to the matter by Snow Harris, who 

 stated that, within the forty years that ended in 1832, two hundred and 

 fifty vessels had been more or less seriously injured by lightning. The 

 commission somewhat haltingly reported that there was no harm in 

 lightning-conductors, and that it thought the system of protection might 

 be tried. Snow Harris thereupon introduced the plan of nailing a dou- 

 ble set of overlapping strips of copper along the masts. After the adop- 

 tion of this method the conductors were struck by lightning in several 

 instances, but in no case did the vessels suffer any damage. This ex- 

 cellent system was only superseded in the end by the natural result 

 of the introduction of iron vessels, which made the ships themselves 

 efficient conductors in virtue of the principle of their construction. 

 The original idea of Snow Harris was, indeed, to bring the general 

 structure requiring defense as nearly as possible into the same non- 

 resisting state that it would have if entirely composed of metal. He 

 was knighted for his services in 1847, and in 1855 was employed to 

 design the protection of the then new Houses of Parliament at West- 



