824 THE POPULAR SCIENCE MONTHLY, 



In the circumstances which liave been described in speaking of the 

 accident there, a yard-square earth-plate could not have been depended 

 upon to prevent the mischief. The lightning would still have pre- 

 ferred the largely developed root of the gas-mains to any such puny 

 substitute, although such an earth-plate, well bedded in moist ground, 

 might have served all purposes in the absence of so formidable a com- 

 petitor. The condition of safety is that which has been so well stated 

 by Professor Rousseau. The communication of the conductor with the 

 earth must not be inferior to that of any neighboring mass of metal. 

 When the arrangement for the earth connection has been efficiently 

 settled, the conductor may be carried up from it, and this may with 

 equal assurance be done either upon the single-rod system of Gay-Lus- 

 sac or upon the multiple-rod principle of Professor Melsens, so long as 

 the building is of moderate size and of a compact form. But, if the 

 building is of large dimensions and of irregular form, the single con- 

 ductor would of necessity have to assume an approximation to the 

 multiple type, as the main stem is branched out above to bring every 

 gable and turret and pinnacle of the structure under its protection. 

 It is only when it has been completed by a broadly cast net of me- 

 tallic meshes and lines that the old early dogma of the protected 

 area can be now allowed to survive even in the mind of the engineer. 

 When the work of construction has been so far carried out it is still, 

 however, not to be looked upon as complete until the stamp of effi- 

 ciency has been placed upon it by the application of the final test, 

 which the advance of electrical science has now placed in the hands 

 of the constructor. It is the crowning distinction of this system of 

 defense that by a very easy process it can be at once ascertained 

 whether all the arrangements of the engineer have been properly car- 

 ried out. By the employment of the ingenious piece of apparatus 

 which is known as the " Differential Galvanometer," the electrician 

 can in a few minutes ascertain what the resistance is that would be 

 offered between the air-terminal and the earth communication of a con- 

 ductor, if a discharge of lightning fell upon the rod. That resistance 

 must never be left unheeded if it amounts to anything in excess of the 

 quantity which is technically known as two ohms. It is quite possible, 

 indeed, by the exercise of judgment and skill, to reduce the resistance 

 in every case somewhat below that. With a conductor which has re- 

 cently been erected upon the Hall of General Assembly in Edinburgh, 

 it was found at the final test that the earth resistance was only the 0*7 

 of an ohm. But the galvanometer test must not only be applied as 

 the last step of the construction ; it must also be drawn upon from 

 time to time, and at not too distant intervals, to ascertain how far the 

 originally well-conceived and well-executed work is or is not in process 

 of being injuriously affected by the physical agencies that are at all 

 times in antagonistic operation to the constructive efforts of man. The 

 free and frequent use of the testing galvanometer is, indeed, the natu- 



