SERIES C — MEAN DIRECTION OF WINDS. 131 



The following series of maps exhibits to the eye the results contained in the pre- 

 ceding tabular series, and shows by means of the straight arrows the mean direction 

 and rate of progress of the wind in the different regions of the northern hemisphere, 

 as explained on page 9. The direction of the arrow shows the direction of the 

 resultant, and its length the ratio of the progressive to the total motion of the 

 wind, the unit being one inch. That is, if the wind were to blow constantly in one 

 direction, so that the whole motion would be progressive, it would be represented 

 by an arrow an inch in length. 



An interrogation point affixed to an arrow denotes that it is doubtful, either in 

 regard to direction or length, and a double one that it is exceedingly so. One affixed 

 to a dot or number shows that the locality is doubtful. The chief source of uncer- 

 tainty in the resultants represented by the arrows is the fewness of the observations 

 from which they were deduced. The numbers on the maps correspond with those 

 in the series, and will serve as references. 



Plate VII. affords a general synopsis of the whole hemisphere. Every resultant 

 that is at all reliable is represented upon it, either singly, or in combination with 

 others in those sections of country where the stations are too numerous to allow 

 each to be distinctly represented by separate arrows. And in combining different 

 stations, care has been taken to select those having nearly the same latitude, since 

 the investigations show that difference of latitude affects the resultants more than 

 difference of longitude. As thus condensed, a single arrow, in some cases, repre- 

 sents observations for more than a century. 



Plate VIII. contains the United States on a larger scale, sufficient to allow all the 

 separate resultants to be exhibited, except in the Eastern and Middle States, where 

 the stations are so numerous that the scale of the map is yet too small to allow 

 them to be represented except in combination as before. This section is drawn 

 upon a still larger scale on Plate IX. Plate X. contains Western Europe on an 

 enlarged scale. 



