EARTH CONTACT. 



24. In the electric theories it is assumed that the course of the 

 current, when it exists at all, must in all cases be continuous and 

 unbroken from p to x, as it is in fact under the conditions repre- 

 sented in fig. 1, when the ends p and n are in contact. It is 

 therefore assumed that in the case represented in fig. 2, the 

 stratum of the earth which is interposed between p' and n' plays 

 the part of a metallic wire joining these points, and that the current 

 which arrives by the wire P pp' at p flows through the earth, as 

 indicated by the arrow, to n', from whence it flows along the wire 

 n' n x to x. 



It is found also in this case that the existence of the current is 

 independent of the lengths of the wires, which do not affect it 

 otherwise than by diminishing its intensity. Whether the wires 

 are 10 feet, 10 miles, or 100 miles in length, the current still flows 

 from P to p' and returns from n' to x. 



25. Thus, admitting the generally acknowledged principle that 

 the stratum of the earth intervening between^' and n' plays the part 

 of a conducting wire, uniting the ends p' and n' of the wires 

 buried, it will follow that the current at p', though separated, as it 

 may be, by a distance of several hundred miles from the point n 

 of its return to x, finds its way nevertheless through the earth 

 unerringly and instantaneously to that point. 



Of all the miracles of science, surely this is the most marvel- 

 lous. A stream of electric fluid has its source in the cellars of 

 the Central Electric Telegraphic Office, Lothbury, London. It 

 flows under the streets of the great metropolis, and, passing on 

 wires suspended over a zigzag series of railways, reaches Edin- 

 burgh, where it dips into the earth, and diffuses itself upon the 

 buried plate. From that it takes flight through the crust of the 

 earth, and finds its oicn way back to the cellars at Lothbury ! ! ! 



Instead of burying plates of metal, it would be sufficient to 

 connect the wires at each end with the gas or water pipes, which, 

 being conductors, would equally convey the fluid to the earth ; 

 and in this case, every telegraphic despatch which flies to Edin- 

 burgh along the wires which border the railways, would fly back, 

 rushing to the gas-pipes which illuminate Edinburgh, from them 

 through the crust of the earth to the gas-pipes which illuminate 

 London, and from them home to the batteries in the cellars at 

 Lothbury ! 



26. To derive all the necessary instruction from what has been 

 explained above, it will be necessary to distinguish what is 

 essential from what is merely optional, and which admits of 

 modification or change without affecting the result. 



27. It will be seen that the electric fluid is evolved by the 

 combination of three bodies, the zinc, the copper, and the acidu- 



123 



