400 WRITINGS OF JOSEPH HENRY. [1855- 



borhood. Besides this, the irregularity in the motion of the 

 electricity which is thus produced, must on mechanical prin- 

 ciples interfere with its free transmission. Points should 

 therefore be omitted along the course of the rod, since they 

 can do no possible good, and may produce injury. 



We may conclude what we have said in regard to light- 

 ning-rods by the following summary of directions for con- 

 structing and erecting them : 



1st. The rod should consist of round iron, of not less than 

 three-fourths of an inch in diameter. A larger size is pre- 

 ferable to a smaller one. Iron is preferred, because it can 

 be readily procured, is cheap, a sufficiently good conductor, 

 and when of the size mentioned cannot be melted by a dis- 

 charge from the clouds. 



2d. It should be, through its whole length, in perfect me- 

 tallic continuity ; as many pieces should be joined together 

 by welding, as practicable, and when other joinings are un- 

 avoidable, they should be made by screwing the parts firmly 

 together by a coupling ferule, care being taken to make the 

 upper connection of the latter with the rod water-tight by 

 cement, solder, or paint. 



3d. To secure it from rust the rod should be covered with 

 a coating of black paint. 



4th. It should be terminated above with a single point, 

 the cone of which should not be too acute, and to preserve 

 it from the weather as well as to prevent melting it should 

 be encased with platinum, formed b}' soldering a plate of 

 this metal, not less than the twentieth of an inch in thick- 

 ness, into the form of a hollow cone. Usually the cone of 

 platinum, for convenience, is first attached to a brass socket 

 which is secured on the top of the rod, and to this plan there 

 is no objection. The platinum casing is frequently made 

 so thin and the cone so slender, in order to save metal, that 

 the point is melted by a powerful discharge. 



5th. The shorter and more direct the rod is in its course 

 to the earth the better. Acute angles made by bending in 

 the rod and projecting points from it along its course should 

 be avoided. 



6th. It should be fastened to the house by iron eyes, and 



