2 92 The National Geographic Magazine 



cision to the line of least resistance at 

 the moment of the taking of the obser- 

 vations on which the weather chart is 

 founded. In the majority of cases his 

 system locates the place to which the 

 storm center will move during the com- 

 ing 24 hours with considerable accu- 

 racy. It might be improved on by tak- 

 ing into account the rate of change in 

 air pressure at all stations during the 

 two hours preceding the observations, 

 and constructing a hypothetical chart 

 based upon such rate continuing for 12 

 or 24 hours, and then applying the sys- 

 tem to the latter chart instead of the 

 real weather map in the effort to deter- 

 mine the future course of the storm. 



The description of Mr Bowie' s method 

 is told in his own words as follows : 



"Assuming erratic storm movement 

 to be due to unequal pressure distribu- 

 tion, it is manifest that the direction and 

 velocity of storm movement could be de- 

 termined were it possible to obtain cor- 

 rect values that would represent the 

 pressure exerted upon a storm from all 

 directions and the eastward drift of air 

 at high levels that carries the storm with 

 it. Working on this theory, effort has 

 been directed toward obtaining a value 

 that would represent the 24-hour east- 

 ward drift from any given locality. 

 To find this value it has been neces- 

 sary, first, to determine the resultant 

 of the pressure from all directions to- 

 ward the storm center. To represent 

 this pressure from all directions, lines 

 radiating from the storm center to the 

 north, northeast, east, southeast, etc., 

 have been given, after considerable ex- 

 perimental work, a length of one centi- 

 meter for each tenth of an inch increase 

 in barometric pressure along these lines, 

 working with a map the scale of which 

 is 160 miles to an inch, or that of the 

 Washington weather map. The result- 

 ant of such lines, or forces, acting to- 

 ward the storm center, which may be 

 found by the rules governing the poly- 

 gon of velocities, will show the direction 



toward which the unequal pressure is 

 forcing the storm. 



4 ' If the pressure of the air from all 

 directions toward the storm center be a 

 factor in determining the direction and 

 velocity of movement of a storm, it is 

 obvious that this resultant, representing 

 the value of and direction toward which 

 the unequal pressure forces the storm, 

 becomes one of the components that de- 

 termine the storm's path. 



"As the 24- hour movement of any 

 given storm is the measure of the forces 

 that determine that movement, it follows 

 that by using this resultant of pressure 

 toward the storm center as one of the 

 components which cause the storm to 

 move along its path it is possible to find 

 the other component of motion by re- 

 solving a force representing a storm's 

 24- hour movement into its two compo- 

 nents. One of these components, repre- 

 senting the pressure effect, being known, 

 the other component, representing the 

 eastward drift, may be found by the 

 rules governing the parallelogram of 

 forces. If there be a basis for this theory, 

 it must necessarily be that the second 

 component, representing the eastward 

 drift, should have approximately the 

 same direction and value for two or 

 more storms in the same locality for any 

 given month of the year, provided the 

 appropriate value is given the pressure 

 acting toward the storm center from all 

 directions. 



' ' This component has been found for 

 a large number of storms, whose values 

 when charted show an agreement that 

 appears to be more than accidental or 

 merely coincident. 



"Having found the component rep- 

 resenting the 24-hour eastward drift, 

 which component is apparently fairly 

 constant in value for any particular 

 locality from year to year for a given 

 month and the resultant of the pressure 

 exerted on the storm center from all 

 directions, the value of which is a vari- 

 able quantity, it is patent that the direc- 



