PREFACE. 



Fig. 2. 



is as follows : It is assumed that if no such forces existed, the mean direction and 

 relative progress of the wind would 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 parallelogram, 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 direction. Thus, for example, at Amherst, Massa- 

 chusetts, Fig. 2, the resultant for January reads 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 direction N. 73° 13' W. and make its length .30. Also draw A D 

 in the direction N. 69° 42' W. and make its length .36. Complete the parallelogram, 

 and the side AC or B D will show 

 the direction and amount of the de- 

 flecting forces, viz., N. 52° 47' W., 

 .0632 ; or a little more than one-fifth 

 as great as the force which deter- 

 mines the mean annual resultant. 

 This value is given in the tables in 

 the second column from the right 

 under the head of 'Force' of mon- 

 soon influences. 



"Figure 3 shows the same /or seasons, where, as in the case of Easton, Pa., the 

 resultant for the spring is represented by A B, which is S. 63° 23' W., length .230 ; 

 and that for the entire year by A D, 

 N. 74° 45' W., length .248; D B is 

 the monsoon influence, which is from 

 S. 11° 18' E., length .172. For the 

 most part the deflecting forces are ap- 

 proximations, determined, with tole- 

 rable accuracy, by mechanical con- 

 struction upon a large drafting scale, 

 though in a few cases they were com- 

 puted trigonometrically, as in the 

 examples here adduced." * * * 



An inspection of Plate 26 will give 

 a more full illustration of the mode 

 of construction and delineation of 

 these forces, as well as show how 

 their computation afi'orded a ready 

 test of the accuracy of the computa- 

 tions of the resultants from which 



they were derived, for these forces must be in equilibrio, however diverse their 

 separate directions and amounts ; were it not so, the particle of air at the end of 

 the months and seasons that constitute its annual course would not be found at the 

 same point that was indicated by the resultant for the year. 



Fig. 3. 



