214 ELECTRICAL CURRENTS OF THE EARTH. 



ap]iareiitly the same physical qualities, beiiiof, in fact, an arenaceous 

 Ibimatiou, as is commonly the case in the neiglibovhood of rivers. The 

 electrodes of zinc were placed successively at the distance of half a metre, 

 (20 inches,) then of 11 metres, (30 feet,) then of 148, (328 feet,) then of 750, 

 (li,4Gl feet,) then of 1,000, (3,477 feet.) At each of these stations I exca- 

 vated four or five holes, in order to vary in every instance the position 

 of the electrodes. The experiments were made by suc(;essively advanc- 

 ing and then returning' to the same holes ; by stretching tiie skein 

 {mctasi'ja) of cop^ier wire covered v/ith gutta-percha, in conjunction with 

 its head, (?) to one of the electrodes, and then re(;overing the skein and 

 turning back again. The copper wire covered with gutta-percha was at 

 one time stretched upon the ground, at another suspended upon poles, 

 at another buried in the grass. It is superfluous to ad<l that, in making 

 these experiments, all the precautions above described were employed, 

 in order to obtain and ])reserve the homogeneity of the electrodes. 



The result obtained from these experiments, many times rei)eated, 

 with every precaution to secure exactness, was that i)i a mixed circuity 

 formed of a metaJlic line and a stratum of earth, horizontal, or as nearhj 

 so as possible, of a length not greater than a Idlomctre, (3,281 feet,) under a 

 clear slcy,and tvith the air calm, there is no proper current of the earth dis- 

 coverable tvith a galvanometer of 2,000 coils. Yet, in a circuit of this 

 leugth, I have noticed, on days of storm and atmosphei'ic disturbance, 

 sudden deviations under the action of the electric discharges. 



EXPEEBIENTS WITH A MIXED CIRCUIT, OF THE LENGTH OF SIX KIL- 

 OMETRES, IN A STRATUM NEARLY HORIZONTAL. 



With the aid of the corps of engineers, I was enabled to establish on 

 the greaG plain of San Maurizio, distant 22 kilometres (loi miles) from 

 Turin, a i)lain set apart for military maneuvers, two mixed circuits, 

 each of which consisted of a stratum of earth and of a copper wire, 2 

 millimetres (jL of an inch) in diameter, and covered with gutta-percha. 

 One of these wires was stretched in the direction of the magnetic merid- 

 ian, the other in a plane perpendicular to that meridian. Both wires 

 had about the same length, namely, 0,400 metres, (4 miles.) The copper 

 wire was suspended upon small wooden poles, such as are used in tield 

 constructions of telegraphic lines. At the extremities of the two lines 

 a hole was excavated, of a rectangular form, with a depth and length 

 of 2 metres, (7 feet,) and a width of 1 metre, (3 feet;) in the bottom of 

 this hole a cavity, or capsule, such as has been above described, was 

 constructed, having a width and depth of 30 centimetres, (1 foot,) and 

 lined with clay, so as to allow no filtration of water. The four cavities 

 were then filled with the same water, which was that of a copious waste- 

 pipe of one of the canals which traverse the plain ; in this water the 

 porcelain cylinders, with the electrodes of zinc, were immersed. 



The first experiments were directed to a verification of the equal con- 

 ductibility of the two mixed lines. It should be premised that the two 

 metallic lines, north-south and east-west, were interrupted about mid- 

 way, and entered at that point into a small chamber, where I had sta- 

 tioned the galvanometers. In the greater number of the experiments I 

 used a galvanometer of 1,500 coils, with an astatic system ; unluckily, 

 this instrument sustained some injuries in being transported, when the 

 experiments were finished, to Turin, so as to be no longer capable of 

 being operated with. 



To measure the conductibility of the two mixed circuits, I caused the 

 current of a good Daniell's battery to pass, first in one, and then in the 



