SOME WORLD'S WEATHER PROBLEMS 207 



astonishing how little is known about the general movements 

 of this immense ocean of air. This perhaps is more our mis- 

 fortune than our fault, because we live at the base of this aerial 

 ocean, like flat-fish live at the bottom of the sea. It is only very 

 occasionally that we have been able to gather information about 

 the upper air currents which play such an important part in 

 the circulation of our air. From the observation of the move- 

 ments of very elevated clouds, and the drift of fine dust ejected 

 into the air by volcanic outbursts, such as Krakatoa, some notion 

 has been obtained of the directions of movements of the higher 

 currents. 



The time has now fortunately arrived when soundings in 

 the air can be made nearly as easily as those in the ocean of 

 water. This is not accomplished by employing manned 

 balloons, for the heights to which these can attain are very 

 restricted. Modern air-sounding machines are of two kinds, 

 and are known as ballons sondres (free balloons) and kites. 

 The former are small hydrogen balloons launched into the air, 

 with their compact set of self-registering meteorological instru- 

 ments, and allowed to wander where the aerial currents please 

 to take them. Kites, on the other hand, are not quite such 

 free agents, for, like the ocean-sounding machine of Lord Kelvin, 

 they are controlled by pianoforte wire, operated by a winding 

 engine. 



In the hands of Messrs. Laurence Rotch, Helm Clayton, 

 Tesserenc de Bord, Assmann, Dines, and others, great heights 

 have already been reached by these means ; and it is possible 

 now to obtain a large amount of very valuable data to a 

 distance of about 14 kilometres, or about 9 miles, from the 

 earth's surface. 



Perhaps one of the greatest advances in meteorology during 

 the present century will be the knowledge gained by the 

 sounding of the air by these and possibly other means ; and 

 since this method of research is only in its infancy, very much 

 greater elevations will no doubt be brought within the range 

 of routine investigation. 



For the present, therefore, we must still be content, like 

 flat-fish, to consider only the thin stratum in our immediate 

 neighbourhood, and for a time to make the best use of the 

 meteorological data gathered in this, the bottom of the aerial 

 ocean. 



