Lightning
the dazzling light emitted by a large spark darting from clouds charged with electricity. In the lower regions of the atmosphere it is white, in the upper one somewhat violet, as is the spark of an electrical machine in a vacuum.
In the upper regions of the sky, where the air is rarefied, it in a vacuum. In the upper regions of tends to take the form of sheet-lightning, sometimes called heat lightning; in the lower regions it becomes more concen- sometimes called heat lightning; in the trated and moves in lines. As a rule lightning strikes objects from above, though occasionally ascending lightning lightning strikes objects from above, has been seen. When it sinks deeply into the ground it sometimes vitrifies the When it sinks deeply into rocks, producing fulgurites. Light seen the instant it occurs. Thunder, travels with such speed that a flash is which is simply the noise of the exploseen the instant it occurs. which is simply the noise of the explo- Thunder, sion, takes about five seconds to travel a mile, hence the distance of any flash, followed by thunder, may be calculated.
If a mile distant, the danger is but slight. a contrivance used in telegraphy for guarding LIGHTNING ARRESTER, a conagainst passage of atmospheric elecagainst passage of atmospheric elecline wires are attached to a plate of line wires are attached to a plate of brass, usually serrated on the under side.
This plate rests on another plate connected with the ground, the two being material. material.