34 6 SCIENCE PROGRESS 



his machine and put him in possession of information which 

 will enable him to distinguish between the situations that are 

 manageable with skill and those that are not. The first airship 

 that had to face a line-squall was wrecked. The proper advice 

 to an airship about a line-squall is to avoid it. 



It is therefore useful for those who are interested in flying or 

 floating to know what are the atmospheric analogies of the tide, 

 the current, the wave, the whirlpool and the bore. The know- 

 ledge is difficult to attain because one cannot see the variations 

 of level and of position of the moving air as one can those of 

 the surface of the water ; moreover, it is not merely the surface 

 layer that we have to deal with. Air craft may be navigated at 

 any height up to, say, 10,000 ft. We are only just beginning to 

 know what goes on in the atmosphere within the lowest two miles. 



I propose to arrange what we have found out about the 

 atmosphere that may be of assistance to airmen in their difficult 

 enterprise under the headings " The Structure of the Atmo- 

 sphere " and " The Texture of Air Currents." Under the first 

 heading I will deal with the different currents that may be met 

 with at different heights ; the modifications of the currents of 

 the lower strata up to, say, 1,500 ft. attributable to the general 

 effect of the friction of the ground; and the local effects of 

 obstacles of sufficient dimensions to cause turbulent motion in 

 the lowest layer. If we picture to ourselves a great current of 

 water flowing over a rocky bed, we may visualise a bottom layer 

 that has to find its tortuous way round obstacles of various 

 sizes and shapes ; above that a stratum where the direct effect 

 of the bottom becomes gradually less and less marked ; 

 ultimately, the remainder of the 10,000 ft. of thickness, where 

 the effect of the bottom is lost or only appreciable by com- 

 paratively slow and regular changes. When we get to these 

 elevations, we must be prepared to find strata which move under 

 general conditions different from those of the lower layers. We 

 may have to deal with currents different in direction as well as in 

 speed, because the forces which control the motion are different. 



There are currents of the atmosphere, analogous in many 

 ways to the currents of the ocean or the river but different 

 altogether from them in the speeds which may be reached. 

 We may expect air currents, high up, to reach a speed of more 

 than 100 miles an hour, a speed which in water would be indeed 

 a serious obstacle to successful navigation. 



