Page:The National Geographic Magazine Vol 16 1905.djvu/328
2 92 The National Geographic Magazine
cision to the line of least resistance at the moment of the taking of the observations 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 coming 24 hours with considerable accuracy. It might be improved on by taking 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 system to the latter chart instead of the real weather map in the effort to determine 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 distribution, it is manifest that the direction and velocity of storm movement could be determined were it possible to obtain correct 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 eastward drift from any given locality. To find this value it has been necessary, first, to determine the resultant of the pressure from all directions toward 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 experimental work, a length of one centimeter 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 resultant of such lines, or forces, acting toward the storm center, which may be found by the rules governing the polygon 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 determine 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 resolving a force representing a storm's 24- hour movement into its two components. One of these components, representing 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 representing 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 variable quantity, it is patent that the direc-