the Unipolar Induction of the Earth. 505 



It is probable that the electric tension is modified to a 

 certain extent by the influence of the clouds near. In general 

 the upper clouds possess, as explained above, a higher electric 

 tension than the lower nebulous layers, and the free positive elec- 

 tricity contained in the last is increased by this tension. When, 

 at the moment of liquefaction of the aqueous vapour, the free 

 positive electricity of these clouds is disruptively discharged, 

 the negative electricity produced by the induction of the upper 

 clouds follows the drops of water to the earth, and these in 

 consequence frequently prove to be charged with negative 

 electricity. The intimate connexion between the condensa- 

 tion of aqueous vapour and the disruptive discharge in all 

 cases is shown by an observation which we have repeatedly 

 had occasion to make, and which without doubt is known to 

 most people *, When a thunderstorm bursts in the imme- 

 diate neighbourhood of the place where one happens to be, 

 especially if vertically above one's head, we see first the flash, 

 then immediately after hear the thunder. A few seconds later 

 the fall of rain or hail commences, of longer or shorter duration 

 according to circumstances. If it has already commenced to 

 rain or to hail when the flash takes place, the fall generally 

 becomes much heavier a few seconds later. The same thing 

 is repeated many times during the storm. Nevertheless it 

 happens sometimes that one observes no sudden increase in 

 the fall of rain or hail, although the discharge has taken place 

 close at hand. These cases seem to show that the tension neces- 

 sary for the electric discharge may arise either under the influ- 

 ence of two electrified clouds, or from an actual liquefaction of 

 aqueous vapour, although this liquefaction may not have pro- 

 ceeded far enough to produce any appreciable increase of rain 

 or hail. This theoretical explanation is in complete accord 

 with observation ; the liquefaction of aqueous vapour is the 

 first operation which takes place, and with it commences the 

 fall of atmospheric water as rain or hail. This liquefaction 

 brings about the condensation of electricity and with it occurs 

 disruptive discharges. The noise produced by the discharge 

 requires a certain length of time to reach the surface of the 

 earth, but the fall of atmospheric water requires a still longer 

 time, and hence these three phenomena are observed in the 

 order indicated. 



The following observation of Armstrong's f may perhaps 

 be considered as an experimental proof that the liquefac- 

 tion of water greatly increases electric tension. In the well- 

 known hydroelectric machine invented by him, the vapour of 



* See the remarks on this point in Herschel's < Meteorology/ p. 131. 

 t Phil. Mag. (3) vol. xxiii. p. 196 (1843). 



