292 Mr. J. Hopkinson on an unnoticed Danger in 



made to see a system of arrangement in the Pacific islands, 

 instead of a " labyrinth ; ,J> to appreciate the vast length of the 

 island-chains in the great ocean with their many parallelisms, 

 and the accordant relations subsisting between them and long- 

 lines of atolls. I was thence led to observe the corresponding 

 system in the features of the continental lands, and the more 

 fully so when afterwards it was proved that Geology was not 

 in America merely the study of strata and fossils, but of the 

 successive stages in a growing continent. Thus a conception 

 of the earth as a unit became early implanted, and the idea 

 also of its development as a unit under movements as compre- 

 hensive as the system in its feature-lines. My faith in any 

 mountain-making theory hitherto proposed is weak. But 

 that idea of system in structure and progress stands, and, 

 however much ignored by students of the earth's stratigraphy, 

 it must have its explanation in a true theory of the earth's 

 dynamics. 



XXX. On an unnoticed Danger in certain Apparatus for Dis- 

 tribution of Electricity. By J. Hopkinson, F.R.S* 



MANY plans have been proposed, and several have been 

 to a greater or less extent practically used, for combi- 

 ning the advantage of economy arising from a high potential 

 in the conductors which convey the electric current from the 

 place where it is generated with the advantages of a low 

 potential at the various points where the electricity is used. 

 A low potential is necessary where the electricity is used ; 

 partly because the lamps, whether arc or incandescent, each 

 require a low potential, and partly because a high potential 

 may easily become dangerous to life. Amongst the plans 

 which have been tried for locally transforming a supply of 

 high potential to a lower and saf^r, the most promising is by 

 the use of secondary generators or induction-coils. It has 

 been proved that this method can be used with great economy 

 of electric power and with convenience ; under proper con- 

 struction of the induction-coils it may also be perfectly safe. 

 It is, however, easy and very natural so to construct them 

 that they shall be good in all other respects but that of safety 

 to life — that they shall introduce an unexpected risk to those 

 using the supply. 



In a distribution of electricity by secondary generators, an 

 alternating current is led in succession through the primary 

 coils of a series of induction-coils, one for each group or 

 system of lamps. The lamps connect the two terminals of the 



* Communicated by tbe Author. 



