H2 WINDS OF THE NOKTHERN HEMISPHERE. 



SERIES P. 



This Series of Tables shows the mean direction of the wind, and the rates of its 

 progress, for each month of the year, at the several places and sections of country 

 mentioned, and hence the annual curve which it describes.^ At a few places, there 

 is added also the average number of days that the winds from the different points 

 of compass prevail in each month ; and, at a number of others, the direction and 

 amount of the forces which deflect the wind from its mean annual direction. The 

 method by which the latter were found was as follows : It was assumed that if there 

 were no forces to deflect the winds, the mean direction and rate of progress w^ould 

 be the same for each month of the year, and equal to one-twelfth of the mean 

 annual progress. If, therefore, according to the usual method of applying the 

 " parallelogram of forces," we make the progress in any month the diagonal of a paral- 

 lelogram, and one-twelfth of the mean annual progress one of the sides, either of 

 the contiguous sides will represent the deflecting force, both in quantity and direc- 

 tion. Thus, for example, at Amherst, Massachusetts, the resultant for January is 

 N. 69° 42' W. 36, and for one-twelfth of the mean for the year, measured on the 



same scale, N. 73° 13' W. 30. Draw A B in the 

 :^^^^J^C[E32___B direction N. 73° 13' W. and make its length 30. 



'~^^S~X Also draw A D in the direction N. 69° 42' W. and 



make its length 36. Complete the parallelogram, 

 and the side A C or B D will show the direction and amount of the deflecting 

 force, viz. N. 52° 47' W. 0.32. For the most part, the deflecting forces are merely 

 approximations, determined, with tolerable accuracy, by construction upon a large 

 scale, though in a few cases they were computed trigonometrically. 



' lu computing the annual curves, it became necessary to fix upon some general principle, upon 

 which to compare and combine the observations taken at different places, and I adopted that of allowing 

 equal weight to the observations of each month, without reference to the manner in which they were taken. 

 There is no doubt that more reliance should be placed upon observations taken several times a day and 

 recorded for sixteen or thirty-two points of the compass, than upon those taken less frequently and 

 recorded less minutely, but it was difficult to decide how much. 



