182 Uature'a 



There is no doubt but that wireless teleg- 

 raphy will some time play an important part 

 in many spheres of usefulness. 



There is another mode (already referred 

 to) for transmitting signals electrically with- 

 out wires through the earth instead of through 

 the air, but in this case it is not through the 

 medium of induction, but conduction. It lias 

 been explained in former chapters that earth- 

 currents are constantly flowing from one point 

 to another where the potentials are unequal. 

 Sometimes these inequalities of potential are 

 caused* by heat and sometimes by electricity, 

 as in the case of a thunder-storm. If a cloud 

 is heavily charged with positive electricity, 

 say, the earth underneath will have an equal 

 charge of negative electricity. Let us illus- 

 trate it by the tides. As the moon passes over 

 the ocean it attracts the water toward it and 

 tends to pile up, as it were, at the nearest point 

 between the earth and the moon. Suppose 

 that (while the water is thus piled up at a 

 point under the moon) we could suddenly sus- 

 pend the attraction between the earth and the 

 moon the water would begin immediately to 

 flow off by the force of gravitation until it had 

 found a common level. Suppose in the place 

 of the moon we have a cloud containing a 

 static charge of positive electricity it at- 

 tracts a negative charge to a point on the 

 earth nearest the cloud. If now a discharge 



