104 



DYNAMIC METEOROLOGY AND HYDROGRAPHY. 



The first chart of fig. 15 gives the pressure at sea-level. It is derived from the 

 isobaric chart of the Weather Bureau, only changed by table 8 a of the Appendix 

 from inches of mercury to millibars. The two charts of pressure-differences (fig. 

 16), from level o to that of 1000, and from this level to that of 2000 dynamic meters, 

 are drawn from the figures in table Q^ The curves of these charts are dotted from 

 the point where the lower limiting surface of the sheet cuts the ground and stop 

 where the upper surface cuts the ground, the part of the earth rising above this 

 upper surface being shaded. The second chart of absolute pressure (fig. 15) is 

 obtained by graphic addition of the first charts of fig. 15 and the first of fig. 16, and 

 in the same way the third chart of absolute pressure is obtained by graphic addi- 

 tion of the second chart of fig. 15 and the second of fig. 16. 



Table Q. Pressure (nt-bars"), in standard level surfaces, and differences of pressure between 



them. United Stales, September 2J, i8p8. 



The three charts of absolute pressure show barometric depressions in the region 

 about the Great Lakes. Interpreting the charts of pressure differences as charts 

 of mass distribution, we get mainly the same results as from the corresponding 

 charts of relative topography. Where these indicate a minimum of specific volume, 

 those of relative pressure give a maximum of density, and vice versa. 



Fig. 18 is a vertical section showing the profile curves of the level and the 

 isopycnic surfaces. As in the corresponding section of fig. 17, these profile curves 

 are drawn not from the simultaneous kite ascents at different stations, but from the 

 successive ascents at Blue Hill. Thus if the cyclone has passed without undergo- 

 ing any interior change (which can not be asserted), these profile-curves will cor- 

 respond exactly to the same atmospheric state as that represented by the chart. 

 But the construction of the section in this way was of special importance in enabling 

 us to find graphically the absolute and relative pressures in the column Blue Hill 

 in table O. 



71. Example 2. Atmospheric Conditions over Europe, November 7, 1901. 



In further illustration we shall consider a second example. It will differ from the 

 preceding one by the greater completeness of the observations. On the one hand, 

 the ascents have reached much greater heights, and on the other the observations 

 from numerous stations at the earth's surface have been available to us in their 

 original form, and not only after reduction to sea-level. 



