Mean Direction of the Wind. 



237 



If we combine the northings and southings, eastings and westings, 

 for the several months, so as to obtain the hititudc and dejiarture for 

 the entire year, and hence compute the mean direction for tlie vear, 

 we shall obtain for a result N. 51°-7 W. If we take the arithmetical 

 mean of the twelve monthly directions, we shall obtain N. 74° W. 

 The large difference arises from the greater uniformity in the wind's 

 dii-ection during the colder months, when the direction is most 

 northerly. 



The numbers in Table II, part 2, were obtained as follows : After 

 computing the mean direction of the wind for each hour, as described 

 on page 2 If, the absolute length of the line indicating its direction 

 Vi'as cojnputed, and this nundDer was divided by the number of the 

 ol)servations for that hour without regai'd to direction. These result- 

 ing numbers, therefore, represent the ratio of the wind's progress, in 

 the mean direction to its entire motion ; and a comparison of these 

 luimbers shows at what hour the direction of the wind was most 

 unifoi'm, and at what hour it was most variable. 



Table III. 

 Mean Direction of the Wind. 



For the entire year, the average change in the direction of the 

 wind from 9 a. m, to 3 p. m., is at Hudson f8°-3; while at Wallingford 

 it is 54°-f , or three times as great as at Hudson. Moreover, at Hudson 

 the direction at 3 p. m, is always more northerly than at 9 a. m., while 

 at Wallingford it is always more southerly. These fiicts seem to 

 indicate that the cause of the diurnal change at Wallingford, must be 

 quite different from what it is at Hudson. 



The Philadelphia observations employed for comparison were those 

 made at the Girard College Observatory* in 1842. The results are 

 shown in the first part of Table IV, while in the second part of the 



* Ma-inetic and Meieornlopfical Obseryations. (xirard Colleji-e. Pliiladelpliia, 1840-45. 



ItiA 



