724 Mr. C. J. P. Cave [April 11, 



To show the relation of the different types of wind strncture to 

 the surface pressures, a model has l)een prepared ; on the map are 

 shown a depression and an area of high pressure, with arrows to show 

 the wind directions ; above the map is a sheet of glass to represent 

 the first 5 kilometres of the atmosphere ; ou this are marked the 

 winds one would expect to meet with at this level under the conditions 

 of pressure supposed ; al)ove this sheet of glass is another representing 

 the thickness of the atmosphere from the 5 kilometre level to the 

 stratosphere. The model is on the scale of one-miUionth, the vertical 

 scale through the glass being approximately the same as the horizontal 

 scale. 



The churning up of the air resulting from the heating of the 

 surface layers by contact with the earth heated daily by the sun, 

 does not presumably reach into the stratosphere ; there being no 

 vertical movements we should expect to find only such horizontal 

 movements as are consistent with a suitable distribution of density. 

 In the simplest cases the wind increases in velocity till a maximum 

 is reached just below the stratosphere ; above this the wind begins 

 to diminish, and sometimes falls off in a very marked manner. There 

 are occasions when all real wind seems to have ceased, and the balloon 

 as it ascends through this curious region moves first in one direction 

 and then in another, so that the relation of wind direction to height 

 can only be represented on a diagram by a disconnected series of 

 points. 



What takes place still higher ? Does this region of calm extend 

 to the very confines of the atmosphere ? We have practically no 

 evidence to go on. In February, 1909, a meteor left a magnificent 

 streak which was visible for two hours and a half ; this trail, which 

 was some 40 miles above the surface of the earth, moved in a manner 

 suggesting very high wind velocities, with sudden variations in the 

 different layers through which it passed. But it is possible that the 

 streak of a meteor may partake of the nature of an aurora, whose 

 luminous patches sometimes move in a remarkable way, and probably 

 under forces other than those of the winds. 



Having for purposes of classification divided the wind structure 

 of the atmosphere into different classes, I must now attempt to put 

 them together, and to show that some of the types that seem very 

 different are in reality closely connected. 



Following on enquiries made by Mr. W. H. Dines on the correla- 

 tion between the surface pressure and various meteorological elements 

 at a height of 9 kilometres, it w;;s suggested by Dr. W. N. Shaw. 

 F.R.S. that the changes of pressure to which our changes of weather 

 are due, have their origin, not near the surface of the earth as 

 hitherto supposed by many meteorologists, but just below the level 

 of the stratosphere at a height of 9 kilometres or so al)ove the 

 surface. This view is in accordance with the observed facts of the 

 wind distribution in the different layers of the atmosphere. 



