14 



DYNAMIC METEOROLOGY AND HYDROGRAPHY. 



set of such chords are seen, drawn to determine the average velocities in the corre- 

 sponding sheets. 



If we wish to have the air-motion represented by specific momenta instead of 

 by velocities, the direct way of proceeding will be to change the velocities contained 

 in column 4 of table A into specific momenta, multiplying them by the corresponding 

 densities of the air. Afterwards the construction is performed exactly as in 

 the case of the other vector, velocity. We multiply the numerical values of the 

 specific momenta by the corresponding thicknesses of sheet Az, and draw in succession 

 segments of line having the directions given in column 3 and the lengths represented 

 by these products. By use of the curve thus obtained we form the vector-average 

 for any sheet precisely as in the case of velocities. 



Table A. Horizontal velocity oj vise 

 long. g io'E.), July 25, 



ed balloon in different sheets. Pavia (lat. 45 11', 

 1907, 7 h 33 m 7*48 m a. m., Greenwich. 



103. The Choice of Suitable Atmospheric Sheets. The method of forming the 

 vector-averages being given, we have to settle the choice of sheets for which the 

 averages should be formed in our practical work. Here different ways may be 

 thought of. 



One possibility will be to retain the principle used in table A, viz, to use a 

 division into sheets characterized by the motion itself, only trying to reduce the 

 number of sheets. This method will be very natural if corresponding changes of 

 wind-direction and wind-intensity are found by the simultaneous ascents from 

 other stations, and especially if these changes of motion are connected with changes 

 of temperature or humidity. 



