ATMOSPHERIC ELECTEICITY. 351 



This relation between the tall of rain and the chan^^es in the electrical 

 state of the apparatus was observed by M. Arao^o* and M. Peltier. f 

 Besides, the latter of these two philosophers noticed an electrical fact 

 which preceded the sudden appearance of hoar-frost, and which tends 

 to verify the opinion advanced by hira as to the formation of this 

 body, which he considered, as well as hail, accompanied by electrical 

 discharges. He states that, before the hoar-frost falls, the needle of 

 the rheometer of his apparatus may be seen to deviate a number of 

 degrees, more or less considerable, then all at once retrograde rapidly 

 and pass from the other side of the zero, as is the case during storms 

 before a flash of lightning, and at the moment of its appearance. By 

 means of these indications he ascertained that the approaching fall 

 of small hailstones could be readily foretold. 



The oscillations Avhich are exhibited in the electric state of the elec- 

 trometer, during the fall of rain or of snow, is particularly worthy of 

 the attention of observers, since similar effects are produced in the 

 electrical discharges during storms. AVe may ask, if the indication 

 of this instrument is really produced by the electricity of the rain. 

 Schiibler's observations cannot decide this question. In fact they 

 were made with Volta's electrometer, supplied with a conductor and 

 an ignited body ; now, when this instrument is exposed to the air 

 during the fall of rain it is subjected at once to the influence of the 

 electricity of the air, which is always positive, to that of the electri- 

 city of the cloud from which the rain falls, and which may be positive 

 or negative ; and finally to the electricity of the rain which strikes on 

 it. The following observation of M. Peltier| may throw some light on 

 this subject: " On the 8tli of June," as this observer states, "toward 

 four o'clock in the evening I had a descending current, the rain began 

 about five o'clock. There were several negative and positive alterna- 

 tions in the direction of the current. The water which fell did not 

 change its sign as the current did, it always gave negative indications 

 to the electroscope." 



To ascertain what in the indications of the electrometer is due to 

 the electricity contained in the rain when it comes in contact with 

 the ground, it is necessary to withdraw the instrument from the elec- 

 trical influence of the clouds, as well as that of the air. Perhaps it 

 may be ascertained by receiving the rain into an insulated metal vase, 

 which may be made to communicate with an electrometer without by 

 a stern placed in the inside of an apartment. It is well known that 

 in this case the instrument is protected from the electrical influence 

 of the air, as well as from that of the clouds. 



We have already said that the electricity of the electrometer ac(j[uires 

 during the fall of rain a tension superior to that of the clear sky. If 

 we reter to the observations made by Schiibler, at Stuttgardt, from 

 1811 to 1812, the intensity of the electricity of the rain and the snow 

 presents an annual period, the stronger electricity taking place in the 

 summer, the feebler in the winter. This is what is indicated in the 

 following table, which gives the mean force of the two electricities for 



' Traite de I'Electri. et du Maj^n., par Becquerel, torn. IV, p. 100. 

 fComptes Kendus, torn. Ill, p. 145, 1836, and Le Journal I'Institut, No. 228, May 10, 

 1838. 

 J Comptes Keadus, torn. Ill, p. 145. 1836. 



