328 Tra nsa ctions of the A merican Institute. 



walls, and when properly painted will not present an unsightly 

 appearance. 



The Chairman — They may be pinned the same as conductors — 

 drawing strips of tin over and tacking at both ends. 



Prof. John Phin — Yes, that plan would be as good as any. Upon 

 the perfection of the ground termination the value of the lightning- 

 rod chiefly depends. If this be defective, no other good features can 

 possibly make up for it. And yet, so little is it understood, that a 

 careful examination of a very large number of rods leads us to believe 

 that fully one-half the lightning-rods in existence are defective in this 

 respect, and consequently furnish but an insufficient protection. The 

 rules to be observed are : 1. The end of the rod ought to be made to 

 terminate in a layer of soil that is permanently wet ; and, 2. The end 

 of the rod ought to expose to this soil as large a surface as possible. 

 Permanently moist earth is to be attained only at considerable depth — 

 say at the level of the water in the wells in the vicinity. Unless we 

 reach this point, we can never be sure that our rod does not terminate 

 in dry or but slightly moist soil. 



The Chairman — Would it not be well to bury pig iron at the ter- 

 minus of the rod % In reconstructing my country house, I found my 

 predecessor had thus disposed of fully two cart loads of odds and ends. 



Prof. Phin — The idea is a good one, or coke from the gas-houses 

 would answer equally well. 



Mr. Slipper — What area of surface is protected by a single rod? 



Prof. Phin — The French Academy sets it down that a rod protects 

 a circle having a radius twice the length of .the rod above the build- 

 ing. But that idea is exploded. It makes no difference how far the 

 rod extends skyward. It is only necessary to have one connection 

 with the earth, but it is indispensible that this connection be a perfect 

 one. I would run rods along the ridge-pole up each chimney, but let 

 them all connect with the main. 



Mr. J. B. Lyman — What is meant by having the rod end in per- 

 manent moisture ? Suppose I am on a hill where I have to dig forty 

 feet for water; must I carry the rod to the same depth? 



Prof. Phin — Not necessarily; the sand may make a good enough 

 material for the rod to end in. 



Mr. R. J. Dodge — How about galvanized rods? 



Prof. Phin — They are no better than painted ones. 



The Chairman — We are certainly greatly obliged to the gentleman 

 for the information he has imparted. I have got some new notions. 

 For instance, I formerly supposed that a painted rod was worse than 



