880 



LESSONS IN ELECTRICITY. 



Fio. 55. 



Paul's Cathedral was first protected. 



The most decisive evidence in favor of 

 conductors was obtained from ships ; and 

 such evidence was needed, to overcome 

 the obstinate prejudice of seamen. Case 

 after case occurred in which ships un- 

 protected by conductors were singled 

 out from protected ships, and shattered 

 or destroyed by lightning. ' The con- 

 ductors were at first made movable, be- 

 ing ioisted on the approach of a thun- 

 derstorm ; hut these were finally aban- 

 doned for the fixed lightning conductors 

 devised by the late Sir Snow Harris. 

 The saving of property and life by this 

 obvious outgrowth of electrical research is 

 incalculable. 



31. TJte Returntny Stroke. 



In the year 1779 Charles, Viscount Ma- 

 hon, afterward Earl Stanhope, pub- 

 lished his " Principles of Electricity." 

 On the title-page of the book stands the 

 following remark : " This treatise com- 

 prehends an explanation of an electrical 

 returning stroke, by which fatal effects 

 may be produced even at a vast distance 

 from the place where the lightning 

 falls." 



Lord Mahon's experiments, which are 

 models Df scientific clearness and pre- 

 cision, will be readily understood by ref- 



erence to the principles of electric induc- 

 tion, with which you are now so familiar. 

 It need only be noted here that whenever 

 he speaks of a body being plunged in an 

 " electrical atmosphere," he means that 

 the body is exposed to the inductive ac- 

 tion of a second electrified body, which 

 latter he supposed to be surrounded by 

 such an atmosphere. 



A few extracts from his work will gir* 

 a clear notion of the nature of his dis- 

 covery : 



" I placed an insulated metallic cylin- 

 der, A B, fig. 55, within the electrical at- 

 mosphere of the prime conductor [p c] 

 when charged, but beyond the striking 

 distance. The distance between the near 

 end A of the insulated metallic bodj 

 and the side of the prime conductor 

 was 20 inches. The body A u wsa 

 of brass, of a cylindrical form, .18 inch- 

 es long, by two inches in diameter. 

 I then placed another insulated brass 

 body E F, 40 inches long by about 3j 

 inches in diameter, with its end E at ths 

 distance of about one-tenth of an inch 

 from the en-l B of the other metallic 

 body A B. I electrified the prime con- 

 ductor. All the time that it was receiv- 

 ing its plus charge of electricity there 

 passed a great number of weak (red or 



