TRANSACTIONS OF THE SECTIONS. 2J 



times called, by attributing it to mutual electro-magnetic induction between different 

 portions of the coil, and anticipated that no such reversal could ever be found in a 

 submerged cable. The effect of this induction is to produce in those parts of the 

 coil first influenced by the motion of the key, a tendency for electricity to flow in the 

 same direction as that of the decreasing current flowing on through the remoter parts 

 of the coil. Thus, after the first violence of the back flow through the key and gal- 

 vanometer, the remote parts of the cable begin, by their electro-magnetic induction 

 on the near parts, to draw electricity back from the earth through the galvanometer 

 into the cable again, and the current is once more in one and the same direction 

 throughout the cable. The mathematical theory of this action, which is necessarily 

 very complex, is reserved by the author for a more full communication, which he 

 hopes before long to lay before the Royal Society. 



On the Necessity for incessant Recording, and for Simultaneous Observations 

 in different Localities, to investigate Atmospheric Electricity. By Professor 

 W. Thomson, LL.D., F.S.S. 



The necessity for incessantly recording the electric condition of the atmosphere 

 was illustrated by reference to observations recently made by the author in the island 

 of Arran, by which it appeared that even under a cloudless sky, without any sensible 

 wind, the negative electrification of the surface of the earth, always found during 

 serene weather, is constantly varying in degree. He had found it impossible, at any 

 time, to leave the electrometer without losing remarkable features of the phenomenon. 

 Beccaria, Professor of Natural Philosophy in the University of Turin a century ago, 

 used to retire to Garzegna when his vacation commenced, and to make incessant 

 observations on atmospheric electricity, night and day, sleeping in the room with his 

 electrometer in a lofty position, from which he could watch the sky all round, 

 limited by the Alpine range on one side and the great plain of Piedmont on the other. 

 Unless relays of observers can be got to follow his example, and to take advantage 

 of the more accurate instruments supplied by advanced electric science, a self-record- 

 ing apparatus must be applied to provide the data required for obtaining knowledge 

 in this most interesting field of nature. The author pointed out certain simple and 

 easily-executed modifications of working electrometers, which were on the table 

 before him, to render them self-recording. He also explained a new collecting ap- 

 paratus for atmospheric electricity, consisting of an insulated vessel of water, dis- 

 charging its contents in a fine stream from a pointed tube. This stream carries 

 away electricity as long as any exists on its surface, where it breaks into drops. 

 The immediate object of this arrangement is to maintain the whole insulated con- 

 ductor, including the portion of the electrometer connected with it and the connect- 

 ing wire, in the condition of no absolute charge ; that is to say, with as much posi- 

 tive electricity on one side of a neutral line as of negative on the other. Hence the 

 position of the discharging nozzle must be such, that the point where the stream 

 breaks into drops is in what would be the neutral line of the conductor, if first per- 

 fectly discharged under temporary cover, and then exposed in its permanent open 

 position, in which it will become inductively electrified by the aerial electromotive 

 force. If the insulation is maintained in perfection, the dropping will not be called 

 on for any electrical effect, and sudden or slow atmospheric changes will all instan- 

 taneously and perfectly induce their corresponding variations in the conductor, and 

 give their appropriate indications to the electrometer. The necessary imperfection 

 of the actual insulation, which tends to bring the neutral line downwards or inwards, 

 or the contrary effects of aerial convection, which, when the insulation is good, gene- 

 rally preponderate, and which in some conditions of the atmosphere, especially during 

 heavy wind and rain, are often very large, are corrected by the tendency of the dropping 

 to maintain the neutral line in the one definite position. The objects to be attained 

 by simultaneous observations in different localities alluded to were, — 1. to fix the 

 constant for any observatory, by which its observations are reduced to absolute mea- 

 sure of electromotive force per foot of air ; 2. to investigate the distribution of 

 electricity in the air itself (whether on visible clouds or in clear air) by a species of 

 electrical trigonometry, of which the general principles were slightly indicated. A 

 portable electrometer, adapted for balloon and mountain observations, with a burn,. 



