i6 



DYNAMIC METEOROLOGY AND HYDROGRAPHY. 



sheets is evident. Thus level sheets of 500 or of 100 dynamic meters, or isobaric 

 sheets of 50 or of 10 m-bars may be used, especially in the lowest strata near the 

 ground, where the greatest irregularities occur. 



104. Use of Standard Level Sheets. Fig. 33 shows the construction leading 

 from the observations given in table A to the average velocities in standard level 

 sheets. The curve having been constructed as described in section 102, points 

 are marked on it corresponding to the heights of 1020, 2040, 3060, 4080 . . . 

 meters, i. e., to 1000, 2000, 3000, 4000 . . . dynamic meters. Then the chords 

 are drawn, and the numbers representing their directions found by use of the divided 

 sheet described in section 98. These numbers are given in column 4 of table B. 

 Further, the length of the chords is measured and the numbers representing these 

 lengths divided by 1020, which represents the common thickness of the level sheets. 

 The velocities found in this way are given in column 5 of table B . 



As the balloon carried self-recording instruments, the pressures in the standard 

 level surfaces have been calculated (Statics, sees. 53-54) and are given in column 2 



Table B. Average horizontal motion in standard level sheets. Pavia (lat. 45 11', 

 long. qio' E.),July 25, 1907, 7 b 33'" 7 h 48 a a. m., Greenwich. 



of table B. The difference between these pressures multiplied by io -5 gives the 

 average density of the air in the different level sheets (Statics, sections 38 and 54). 

 These densities appear in column 3 and give full information on the masses moving 

 with the velocities specified by the columns 4 and 5. 



To get the corresponding specific momenta, we can simply multiply the average 

 velocities, column 5, by the corresponding densities, column 3. The result is noted 

 in column 6. 



Regarding the specific momenta obtained in this way, it should, however, be 

 emphasized, that they are not identical in direction and intensity with those average 

 momenta which would be found if we multiplied each velocity given in table A by 

 the corresponding density and repeated the construction for forming the vector- 

 average. But on account of the relatively slow decrease of density with the height, 

 the difference will generally be insignificant in comparison with the unavoidable 

 uncertainty of the observations of velocity. 



105. Use of Standard Isobaric Sheets. Fig. 34 shows the construction lead- 

 ing from the observations given in table A to the average velocities in standard 

 isobaric sheets. The curve is precisely the same as that of fig. 33. But now we 



