Secchi on Magnetic and Atmospheric Perturbations. 369 



The immediate cause of the connection thus traced, M. 

 Secchi ascribes to atmospheric electricity, which, when dis- 

 charged from the air to the earth, must generate strong currents 

 by which the needle is affected. Such currents, he observes, 

 exist not only during auroral manifestations, but also during 

 storms, and are exhibited by each instrument according to its 

 nature, the galvanometer showing changes in tension, and the 

 compass-needle making known alterations in the total force of 

 the current which passes beneath it. With reference to the 

 questions of whence comes the electricity circulating in the soil, 

 and what is its immediate vehicle, he replies by pointing to the 

 precipitations from the atmosphere. The rain especially, he 

 says, discharges an immense quantity of electricity into the 

 earth, and, in general, it may be said that strong actions upon 

 the instruments only occur after a rainfall has taken place at 

 some point more or less remote, even beyond the limits of the 

 visible horizon. This circumstance may, perhaps, explain the 

 fact that magnetic perturbations indicate approaching squalls. 

 Rain usually produces negative electricity over a considerable 

 extent of atmosphere, and it is itself generally negative, which 

 accounts for the notable diminution of horizontal intensity 

 which precedes squalls. The precipitation of vapour without 

 rain, which often happens between eight and nine on clear 

 nights, and which is accompanied by very strong electricity, 

 may explain the magnetic perturbations which occur at that 

 time, and the diurnal electric period which corresponds with 

 the movements of the horizontal needle may belong to the 

 same class of meteorological facts. Even the aurora borealis 

 may be included in this category, as there is a continual fall of 

 ice-needles, almost invisible, but whose existence is clearly 

 shown in the narratives of Polar voyages. Atmospheric elec- 

 tricity on these occasions may, perhaps, be exalted by accessory 

 causes, such as the change which takes place when vapour 

 passes to the state of ice, or by the friction of wind against the 

 little icicles in a dry and very insulating atmosphere, and also 

 by the inductive action of superior regions on the falling and 

 floating particles of ice. These various subjects, M. Secchi tells 

 us, are illustrated in his Memoires, but he does not pretend that 

 magnetic disturbances have no other causes than those indicated 

 in the preceding remarks. Further observations will be made 

 at Rome, and notices afforded in the Bulletin Meteorologique, 

 published in that city. 



It is interesting to know that the instruments used by Mr. 

 Secchi were supplied from the Observatory at Kew. 



