Emanation in the Lower Regions of the Atmosphere. 23 



enables us to draw a step in the surface trajectory which 

 passes through the point if we can assume the average move- 

 ment of the air to have remained constant during a sufficient 

 interval. Thus, for example, if an observation gives the 

 wind direction at a station as S.W. and its speed the equi- 

 valent of 20 miles per hour, we may suppose that within the 

 half-hour preceding the observation the air travelled 10 miles 

 from the South- West, and in the succeeding half-hour it 

 travelled 10 miles further towards the North-East. For 

 longer periods a proportionately longer step must be drawn. 



So long as the motion of the air is of considerable 



magnitude and remains persistent for a considerable time, 

 there is little difficulty in drawing the steps with considerable 

 confidence; but when we have to deal with a region of light, 

 airs, and in the outlying region of an approaching depression, 

 or when the changes are rapid, as in the region of the centre 

 of a depression, the drawing of the trajectory is an uncertain 

 process." 



Shaw and Lempfert's air trajectories over the United 

 Kingdom were obtained from charts got out every hour or 

 two hours. The charts at my disposal had successive intervals 

 of 6, 5, 13 hours, so that the trajectories given in Plate II. 

 are not very trustworthy. Still in the absence of anything 

 better they serve a useful purpose. 



Again my " exposures " lasted 21 hours, so that the trajec- 

 tory of the air arriving at the beginning of the exposure 

 might be very different from the trajectory of the air arriving 

 at the end of the exposure. To get over this difficulty I 

 have drawn in some cases trajectories at successive intervals 

 of about a day. The course of the trajectories of the air 

 arriving at Cambridge between the instants taken may thus 

 be roughly approximated to by a consideration of the known 

 trajectories. 



Trajectories of the surface air-currents arriving at Cambridge 

 when a low value of the emanation content was obtained 

 (Maps 1, 2). 



Curves (1), (2), (3) of Map 1 give the three trajectories for 

 the surface air arriving at Cambridge on March 24, 6 P.M., 

 March 25, 7 a.m., March 25, 6 p.m., during the passage of a 

 deep cyclone, the path of whose centre is represented by the 

 dotted line. The velocity of the centre was 20 miles per 

 hour. The wind velocity was from 5 to 7 on the Beaufort 

 scale, i. e. from 15 to 35 miles per hour, so that the cyclone 

 would be said to be a fast traveller. The air represented by 

 curve (2) covered its 320 miles of land travel in 20 hours. 



