EXAMPLES OF ATMOSPHERIC MOTIONS. 1 73 



in North Dakota to the southern coast of Lake Superior, for here it would have a 

 head-wind of 28 meters. But it would easily accomplish the voyage by the circuit 

 south of the center of the cyclone. If we were able to estimate the degree of per- 

 sistency of the state of motion, and the direction in which the changes are to take 

 place, it would be possible by use of such charts to plan the course of aerial ships so 

 that they will reach their destination in the shortest time. 



203. Charts of Acceleration. Charts XXXVII to XLI exemplify the kinematic 

 diagnosis as far as it can be carried out by use of observations taken at one epoch 

 only, and only at the stations at the ground. When we have observations from two 

 successive epochs, we can go one step farther and determine the acceleration of the 

 motion kinematically. 



The charts of plates XXXVII or XXXVIII were taken from the registered 

 values of wind-intensities and wind-directions during the hour from 7 to 8 a. m., 

 time of 75th meridian. The charts of plates XLII and XLIII show the corresponding 

 representation of the motion derived from the values registered during the hour 

 from 10 to 11. As will be seen, the point of convergence has been displaced a little 

 more than 200 kilometers toward northeast, but otherwise the general features of the 

 chart are unchanged. 



In order to determine the average acceleration during the interval of time 

 between the two epochs, we first form the chart of the average velocity for this 

 interval of time. This is done by addition and division by 2 of the two vector- 

 fields represented by plates XXXVII and XLII. The result as obtained directly, 

 represented by isogonal curves and curves of equal intensity, is shown on plate XLIV; 

 plate XLV shows the corresponding representation by lines of flow and curves of 

 equal intensity. 



By the subtraction of the same two vector-fields and division by the interval 

 of time, 3 hours or 10,800 seconds, we form the chart of local acceleration. Plate 

 XLVI contains the representation of this vector by isogonal curves and curves of 

 equal intensity, and plate XLVII gives the representation by vector-lines and 

 curves of equal intensity. 



From charts of average motion (plates XLIV or XLV) we derive the chart of 

 stationary acceleration as described in section 197 (A). The result is given on 

 plate XLVIII by isogons and intensity-curves, and on plate XLIX by vector-lines 

 and intensity-curves. 



The true acceleration of the moving particles is finally obtained by the addi- 

 tion of the vector-fields representing local and stationary acceleration. The result 

 is represented by the charts of plates L and LI, on the first by isogonal curves and 

 intensity-curves, on the second by vector-lines and intensity-curves. 



Much more experience than we have at present must be gained before we can 

 estimate the degree of objective reliability of a chart of acceleration like that given 

 on these plates. In the western mountainous parts, where in many cases great doubt 

 may arise as regards the charts of velocity from which the chart of acceleration has 

 been derived, the values found for the acceleration must of course be used with 



