UPPER TURBULENT ATMOSPHERIC ZONE 221 



meteors plunging into the atmosphere become visible; it is commonly 

 about 80 or 90 miles above the ground; rarely 100 miles. Below that line 

 are two other thin lines, M. T. and M'. T'., the outer one 70 ± miles above 

 the ground, the inner one 40 =h miles. They inclose the zone within which 

 meteorites develop trains in addition to streaks. These trains seem to be 

 sheaths of luminous air that diffuse outwardly from their axes. They 

 remain visible for periods ranging up to an hour, and occasionally, but 

 rather rarely, more. These give visibility to tlie movements of the atmos- 

 phere in this elevated zone, and thus have important meteorological sig- 

 nificance. These movements are quite extraordinary. Not infrequently 

 the currents at different heights flow in diverse directions and the train 

 rapidly becomes zigzag. Sometimes it becomes tortuous, implying much 

 turbulence. The velocities greatly vary and are sometimes very notable, 

 not very infrequently reaching 100 miles an hour; even 300 miles per 

 hour have been authoritatively reported. On the whole, the velocities 

 seem to be higher than those which commonly prevail at lower altitudes. 

 These high velocities, of course, are to be interpreted in the light of the 

 great tenuity of the air. These swift, diverse, and turbulent currents 

 seem to put out of court those mathematical deductions relative to the 

 constitution of the upper atmosphere which are based on the assumption 

 of its quiescence. Unfortunately these have found an unwarrantable 

 place in meteorological literature. 



The surficial diastrophic Zone 



The broad black band of Diagram IV represents the surface of the 

 earth incidentally, but its special purpose is to call attention to a dynamic 

 zone in which the potential resistance to deformation has three distinctly 

 different values in the six cardinal directions — a matter of moment in the 

 study of the earth's surficial diastrophism. Displacement upward en- 

 counters little more than the resistance of the atmosphere and the weight 

 of the material displaced; displacement in the four horizontal directions 

 encounters the much more considerable resistance of the terrane itself, 

 while displacement downward encounters the almost insuperable resist- 

 ance of the solid earth-body. These diversities of potential resistance 

 stand in sharp contrast to the four free sides and two stressed sides 

 usually involved in laboratory experiments. The results of such experi- 

 ments, therefore, need scrupulous qualification in their application to 

 natural diastrophism. These distinctions of potential resistance are not 

 wholly confined to the surficial zone; they are merely most pronounced 

 there; in a declining way, they hold in respect to the deeper horizons. 



