112 DISCOVERY CH. 



thunderstorms, and really reach the earth from outer 

 space. In its annual journey around the sun the earth 

 now and then encounters stray fragments of cosmic 

 matter, and draws them toward itself by the force of 

 gravitation. When the mass reaches the earth's atmos- 

 phere, friction against the air makes it white hot, and 

 like a moth flying into a flame it is consumed, the 

 streak of light thus produced being a shooting star or 

 meteor. Sometimes the piece of cosmic material is so 

 large that it is not completely consumed as it traverses 

 the atmosphere ; and in this case it reaches the earth 

 as a solid mass a meteorite which may weigh a few 

 ounces or several tons. Many of these meteorites are 

 preserved in our museums, but though they may make 

 a noise or a series of explosions as they hurl themselves 

 toward the earth, they are not connected in any way with 

 thunderstorms, and cannot correctly be termed thunder- 

 bolts. 



Other objects often mistaken for thunderbolts are 

 known to geologists as fulgurites, and are produced by 

 the fusion of grains of loose sand by a lightning dis- 

 charge. At the mouth of the river Irt, in Cumberland, 

 fulgurites have been found extending to a depth of forty 

 feet in the sand, and a fulgurite found in a sandy stratum 

 at Macclesfield reached to a depth of twenty-two feet. 

 It is perhaps natural to conclude that tubes or patches 

 of fused rock, found after lightning has been seen to 

 strike the earth in a place where only loose sand could 

 be seen, actually came from the clouds, but here again 

 the view that " seeing is believing " leads to an erroneous 

 conclusion. 



In the absence of any precise knowledge of the nature 

 of globular or ball lightning, it may be undesirable to 

 assert that nothing solid can come from a thunder- 



