Page:Popular Science Monthly Volume 56.djvu/373

Rh the rails and return feeders, and including subterranean water-courses, sewers, and metallic earth veins.

In the light of our experience of the last eight years, it is easy to identify as electrolysis the effects shown in the accompanying cuts of buried metals that have been actually subjected to a flow of electricity. It is not to be inferred that the destructive action here depicted is universal throughout our towns, but, rather, that the damage occurs in spots, its rate of progress being dependent upon the amount of current and the duration of the flow. Dry, sandy soils tend to keep down the flow of current by interposing a high resistance, so that in such localities electrolytic effects are not as pronounced as in wet, loamy soils. In the same way, the character of the pipe surface or coating, if there be any—acts as a partial barrier to check the passage of electricity.

Until recently it was generally supposed that cast iron was not attacked—at least not rapidly enough to cause alarm. In Brooklyn the water mains, of very

 The fibrous appearance of the surface is characteristic of wrought iron and steel.