200 On Rain-Clouds and Atmospheric Electricity. 



changes may be predicted (see our paper on Observations of 

 Atmospheric Electricity, read before the Asiatic Society of 

 Japan, April 25, 1877). And we see from the above equa- 

 tions why this should be the case, since changes in the state 

 of the atmosphere, whether brought about by actual motions, 

 or by alterations of density from cooling, or from other causes, 

 must produce changes in the specific inductive capacity of the 

 dielectric, and consequently alterations of the potential of the 

 earth in the neighbourhood. Assuming dry air at 760 mil- 

 lims. pressure and at 0° C. temperature to have a specific 

 inductive capacity unity, then as we mix some aqueous vapour 

 with it the specific inductive capacity increases and becomes 

 larger than 1 ; and in addition, as some of the vapour con- 

 denses, we know that the globules of water, excessively small 

 at the beginning, soon increase in size, so that, as the specific 

 inductive capacity of water is some millions of times that of 

 air, the mean specific inductive capacity of the space is im- 

 mensely increased ; and hence we see that the cooling of the 

 atmosphere and the formation of clouds, or the approach of 

 clouds, may occasion great changes in the distribution of 

 atmospheric potential at any place, and consequently give rise 

 to strong earth-currents. And if the cloud has no charge of 

 its own, the direction of these currents will be such that posi- 

 tive electricity will flow from the place from which the cloud 

 is passing to that from which it came ; for since the earth is 

 known to be negatively electrified with regard to space, the 

 introduction of a cloud or other dielectric of greater specific 

 inductive capacity than air must make the potential of the 

 part of the earth's surface underneath it less negative than 

 before — that is, must raise the potential. 



Although mere changes in the density of the air will, for 

 the reasons given above, be sufficient to produce earth-cur- 

 rents, still we should imagine that the changes in the atmo- 

 spheric potential commonly observed are due, not so much to 

 the direct change of the specific inductive capacity with 

 change of density, as to the much greater changes that must 

 be produced in the capacity by the formation of clouds result- 

 ing from the change of temperature and density ; so that we 

 should expect that observations of atmospheric electricity will 

 be of greater use in the predicting of rain and snow than of 

 wind-storms. 



As the atmosphere does not altogether consist of non-con- 

 ducting matter, portions of it, especially cloudy portions, are 

 capable of acquiring electric charges, through changes of tem- 

 perature or motions of the atmosphere ; and these portions sub- 

 sequently become more or less conducting through changes of 



