120 



THE POPULAR SCIENCE MONTHLY. 



A Classified Catalogue of the Lepidoptera of 

 Canada. By Alexander Milton Ross. 

 Toronto, 1872. 



Report submitted to the Trustees of Cornell 

 University in behalf of a Majority of the 

 Committee on Mr. Sage's Proposal to en- 

 dow a College for Women. By Andrew 

 D. White. Ithaca, 1872. 



Report on the Climatology and Epidemics of 



Minnesota. By Charles N. Hewit, M. D. 



Philadelphia. 1872. 

 Short-hand and Reporting. A Lecture. By 



Charles A. Sumner, with Appendix. San 



Francisco, 1872. 



MISCELLANY. 



The Ground Connection of Lightning- 

 Rods. It is asserted, by all the later author- 

 ities on the subject of lightning-rods, that a 

 proper ground termination of the rod is of 

 the very first importance to its efficiency as 

 a protection against accidents by lightning. 

 The electricity of the cloud will select the 

 easiest path into the earth, or, as it is tech- 

 nically stated, follow the line of least resist- 

 ance ; and it is to furnish a path less resist- 

 ing than the building itself that the light- 

 ning-rod is erected. But it is not enough 

 that the rod have a sufficient conducting 

 capacity. The current must be able to 

 leave it, at the place where it terminates in 

 the ground, as fast as it passes along the 

 rod, else there is an accumulation, or dam- 

 ming up as it were, in the rod, which, when 

 it has attained a certain volume or intensity, 

 will relieve itself with explosive violence; 

 and thus the appliance becomes an actual 

 source of danger to the building, rather 

 than a means of protection. Mr. David 

 Brooks, in an able paper on " Lightning 

 and Lightning-rods," published in the Au- 

 gust number of the " Journal of the Frank- 

 lin Institute," says on this point : "I do 

 not say that a greater proportion of build- 

 ings having lightning-rods are destroyed or 

 injured than of those not having them, al- 

 though those making careful observations 

 do give that as a result of their statistics. 

 I shall undertake to show that this difficulty 

 consists in the defective connection of these 

 plates with the earth, and also that with a 

 proper connection with the earth they are 



almost, if not an absolute, means of protec- 

 tion." Says Prof. John Phin, in his admi- 

 rable brochure on " Lightning-Rods and how 

 to construct Them : " " Upon the perfection 

 of the ground termination mainly depends 

 the value of the lightning-rod. If this be 

 defective, no other good features can pos- 

 sibly 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 be- 

 lieve that fully one-half the lightning-rods 

 in existence are defective in this respect, 

 and consequently furnish but an insufficient 

 protection." 



All objects may be said to conduct elec- 

 tricity, but they vary greatly in their con- 

 ducting capacity. Copper conducts six 

 times as well as iron, and iron thousands 

 of times better than water, and water again 

 thousands of times better than dry earth. 

 That is to say, a rod of iron, to have the 

 same conducting capacity as a rod of cop- 

 per, would require to be of six times the 

 sectional area, while, if a rod or column of 

 water were employed, it would require to be 

 many thousands of times greater in section- 

 al area than the iron, and dry earth again 

 many thousands of times larger than the 

 column of water. In connecting a rod with 

 the ground, allowance has to be made for 

 this difference in conducting capacity, suffi- 

 cient earth-surface being joined to the rod 

 to give a conducting capacity approaching 

 to or equalling that of the rod. Otherwise 

 the lightning discharge, unable to find a 

 free passage into the ground, accumulates 

 until the tension becomes so great that it 

 bursts from the rod with explosive violence, 

 taking the track which affords the readiest 

 means of escape, and often doing serious 

 damage in its progress. 



Accidents of this character are by no 

 means rare. Mr. Henry Wilde, in a com- 

 munication to the Mechanic s Magazine, 

 gives two cases of fire, resulting from the 

 ignition of the gas by lightning in buildings 

 where it left the conductor and took to the 

 gas-pipes. In one instance, the discharge 

 passed down a wire rope suspended by the 

 side of a tall chimney, and, leaving the 

 lower end of the rope, which was some ten 

 feet from the ground, darted across a space 

 of sixteen feet to a gas-meter in the cellar 

 of an adjoining cotton-warehouse, where it 



