FIELDS OF PRESSURE AND MASS IN THE ATMOSPHERE. IOI 



The iooo m-bar surface rises to a height of ioo dynamic meters or more above 

 the Atlantic Ocean, Hudson Bay, and the nearest parts of the coast; it cuts the 

 earth's surface along a line running approximately parallel to the coast, and shows 

 a marked depression in the region about the Great Lakes, where it goes down to or 

 below sea-level. The 900 m-bar surface runs at an average height of 950 dynamic 

 meters, and cuts the earth along the lower slope of the Rocky Mountains as well as 

 about a few of the higher peaks of the Appalachian Mountains, which rise as islands 

 above the surface. The 800 m-bar surface runs at a height of 1950 dynamic meters 

 and cuts the earth only along the upper slope of the Rocky Mountains. Both show 

 the same depression as the 1000 m-bar surface in the region about the Great Lakes. 



The curves of the first chart, fig. 14, giving the mutual topography of the 900 

 m-bar and the 1000 m-bar surface, run mainly east and west, indicating a decreas- 

 ing distance between the surfaces as we proceed from south to north. The curves 

 of the next chart, giving the mutual topography of the 800 m-bar and the 900 m-bar 

 surfaces, have a very different course, running mainly north and south, and indicat- 

 ing a decreasing distance between the surfaces as we proceed from west to east. 

 Interpreted as charts of the distribution of mass in the standard isobaric sheets, 

 the first shows decreasing specific volume, i. e., increasing concentration of mass, 

 as we proceed from south to north, while the second indicates a corresponding con- 

 centration of mass as we proceed from west to east, the greatest concentration 

 apparently being found a little south of the greatest depression of the isobaric 

 surfaces. 



Fig. 17 is a vertical section showing the profile curves of the isobaric and the 

 isosteric surfaces. This section is not, however, derived from the observations 

 used in drawing the charts, but from the successive ascents performed at Blue Hill 

 on each of the four days September 21 to 24. Supposing the cyclone to have 

 moved during these days without undergoing any change in its interior constitu- 

 tion, the section obtained in this way would have given the same result as a set of 

 simultaneous ascents from four properly chosen stations on any of these days. We 

 waive the question as to the invariability of the cyclone during this time, and there- 

 fore also as to what approximation the four successive ascents from Blue Hill may be 

 interpreted as four simultaneous ascents from different stations. The sections ob- 

 tained by successive ascents from the same stations will always be of value in them- 

 selves, and in this case in enabling us to find by interpolation the state of the 

 atmosphere above Blue Hill at the time of the Weather Bureau ascents September 

 23. The point marked B. H. indicates the vertical of the moving cyclone which 

 was above Blue Hill at the time, and from its sections with the isobaric and the iso- 

 steric curves the numbers under the column Blue Hill in table P have been derived. 



Table Q^contains the result of the same kite ascents as table P, but worked 

 out for the absolute pressures at given levels and the pressure differences from 

 level to level. The corresponding synoptical representation of the state of the 

 atmosphere is given in figs. 15, 16, and 18. The isobaric curves are drawn con- 

 tinuously or dotted according as they represent real pressure in the open air or 

 ideal pressure below the earth's surface. The curves of intersection of the different 

 levels with the earth's surface are drawn heavy. 



