ELECTRICAL CURRENTS OF THE EARTH. 



tlie electrode of zinc sunk in the ground in tlie usual manner, while at 

 the station of Pontedera, where the galvanometer was placed, tlxe above- 

 mentioned wire proceeded to one of the ends of the galvanometer, while 

 the other end was made to communicate with a copper wire covered 

 w"ith gutta-percha which terminated at the earth and united with the 

 other electrode of zinc. 



The following are the numbers obtained from the two experiments, 

 in both of which the constant deviation of the needle indicated the usual 

 ascending current in the wire: 



lu these experiments, especially in that of the night of the 11th of 

 July, the deviation had been constant in the interval between one obser- 

 vation and another, and the variations took i)lace very slowly. But 

 this constancy was not so absolute as that noticed in the iirevious 

 experiments with a short circuit; the needle in these experiments 

 between Volterra and Pontedera haviug constantly exhibited a sort of 

 tremulous oscillation in an arc, which was never greater than one 

 degree. Moreover, as well in the experiments of the lltli as in those 

 of the 23(1 of July, it was observed that, thrice on the former iind four 

 times on the latter night, at various intervals of time, the deflected 

 needle, which had seemed fi.xed, suddenly descended to 0^ and oscillated, 

 oftenest for a few seconds, but on one occasion for about an hour in the 

 opposite quadrant, never taking a tixed direction, and returning by a 

 rapid movement to its stationary position under the ascending current. 

 Tliese extraordinary movements of the needle, I am persuaded, though 

 without being able absolutely to affirm it, were independent of the errors 

 of the experiments, even counting among these the case of a voltaic 

 current introduced for a moment into the circuit at Volterra, where I was 

 not present. 



The last series of experiments which I shall report, relates to observa- 

 tions made on a long telegraphic line from I\T.'ea to Courmayeur, first in 

 October, 1801, and again in November, 18G6. These experiments were 

 made in three different sections, of which that line is composed. The 

 first, between Ivrea and iSt. Vincent, nearly parallel to the meridian, is 

 30 kilometres (22 miles) in length, with a difference of level between the 

 extremities of 281 metres, (922 feet.) The second section between St. 

 Vincent and Aosta is 25 kilometres (10 miles) long, and the difference 

 of level, S3 metres, (272 feet.) The third, between Aosta and Courmayeur, 

 at the extremity of the valley at the foot of Monte Bianco, is 27 kilo- 

 metres (17 miles) long, and the difference of level of the two extremities 

 012 metres, (2,103 feet.) In 1801 the experiments were made separately 

 in the three sections of the line; in tho.se of 1800, only two sections, 

 that, namely, between Ivrea and Aosta, and that between Aosta auc 



