PROTECTION AGAINST LIGHTNING. 685 



scattered into thousands of fragments, without any damage heing done 

 to the vessel itself. A Turkish ship cruising near at the time, with a 

 chain from the masthead which did not reach into the sea, had a hole 

 like that which would have been made by a cannon-shot pierced 

 through the hull near the water-line. The inference was drawn from 

 these cases that chains, and especially small chains, were not trust- 

 worthy for the purpose of conducting discharges of lightning. The 

 mechanical violence sustained was perceived to be due to the circum- 

 stance that the conductors provided were of a bad principle of con- 

 struction. They were at the least from nine to ten times too small. 

 Conductors provided by engineering art are intended to be struck, but 

 struck in such a manner as to govern the lightning, and to render the 

 heaviest strokes harmless. No case had been known of a continuous 

 iron rod, three quarters of an inch in diameter, or with a sectional 

 area of one and a quarter square inch, having been structurally in= 

 jured. The cases alluded to were held to demonstrate that conductors 

 must have a sufficient size and thickness of metal, and must be con- 

 tinuous and without defect from end to end. It was definitely settled 

 that, in accordance with these requirements, a square iron rod used as 

 a defense against lightning should have, at least, a diameter of nine 

 sixteenths of an inch, and that a round rod should have a diameter of 

 ten sixteenths of an inch. 



Some modification was also made in this instruction in reference to 

 air-terminals. It was considered that a blunt point, fashioned like 

 the apex of a cone subtending an angle of thirty degrees, would be less 

 liable to fusion than a sharper and more attenuated point, and that 

 therefore it should be adopted for the upper terminal, although it 

 might, perhaps, not exert altogether so satisfactory a neutralizing in- 

 fluence. The area protected by a conductor was now considered not 

 to be so definite and certain as it was previously held to be. It was 

 recognized that it would be less in the case of a building with a 

 metal roof, for instance, than in other circumstances. The earth con- 

 tact, it was remarked, could not be looked upon as efficacious unless it 

 were made, through the instrumentality of sheets of water, at least as 

 large as the area of the storm-cloud, and access to such sheets must 

 be secured by boring both in the direction of the surface moisture 

 and in that of the deeper soil. Chains of red copper with a square 

 section of three eighths of an inch, and weighing a pound and three 

 quarters per yard, were recommended for ships. Such were the princi- 

 pal suggestions of a practical kind that were submitted in this report. 

 In all other pailiculars the provisions of the earlier instructions were 

 substantially approved and confirmed. There was, however, one inci- 

 dental remark contained in this excellent report which is deserving 

 of the highest commendation and approval on account of its practical 

 wisdom. This emphasized the necessity for continued and minute 

 observation and study of the effects of thunder-storms, with a view 



