3G8 WKITINGS OF JOSEPH HENRY. [1855- 



although it must have been at the time many miles in alti- 

 tude above the surface of the earth. 



Inductive action of the cloud. — A cloud formed as we have 

 described must produce a great inductive effect on the earth 

 beneath, and as it is borne along (from the west in this lati- 

 tude) over the ground, the intensity of the electricity of the 

 lower part must constantly vary, on account of the differing 

 conductive capacity of the materials at or below the sur- 

 face. For example, since water is a better conductor than 

 dry earth, if the cloud is moving in a line that prolonged 

 would cross a river, its course will frequently be changed, and 

 in a similar way we can explain the fact that discharges of 

 lightning more frequently fall on some places than others. 

 Although the cloud may be impelled in the same direction 

 by the wind, yet the attraction of the surface of the water 

 (rendered more than naturally negative by induction,) will 

 tend to draw it from its course. And since the induction 

 acts at a distance through all substances, if a quantity of 

 water or good conducting material exist below the surface of 

 the earth, the cloud will be similarly affected. It frequently 

 happens that when a heavy discharge of lightning passes 

 near a house or descends along a rod, inductive effects are 

 exhibited which are more startling than dangerous. 



We have seen in the experiment described on page 693 (Fig. 

 9,) that an induced spark was exhibited at the edge of a large 

 disc covered with tinfoil, in the lower ^ory, by suddenly 

 drawing the electricity from a similar disc in the upper part 

 of a house. A precisely similar arrangement, but on a much 

 more gigantic scale, is presented when a highly charged 

 thunder-cloud is in the zenith of a building. Now if the 

 intensity of this be suddenly diminished by a discharge to 

 the earth, flashes of electricity and sparks from different ob- 

 jects within the house will be observed. The explanation 

 of this is very easy. The free electricity of the cloud, which 

 we may suppose to be positive, repels all the positive elec- 

 tricity of conductors and partial conductors into the ground, 

 and renders them negative. They will be brought into this 

 state very gradually however, either by the comparatively 



