170 WINDS OF THE NORTHERN HEMISPHERE. 



Plates XL and XII. show the truth of the hiw just stated more clearly than any 

 verbal explanations. The directions and lengths of the arrows show the directions 

 and amounts of the forces which deflect the wind from its mean annual direction 

 in the several months of the j'ear. These arrows are drawn on a scale twelve 

 times greater than those which represent the mean annual directions, in Plates VII. 

 to X. inclusive ; but as the latter represent the mean progress of the wind for the 

 entire year, while the mean monildy progress, if there were no deflection, would be 

 only one-twelfth as great, both may be regarded, for the purpose of comparison, as 

 drawn on the same scale. So that the length of an arrow on one of those plates, is 

 to the length of one for the same place on Plate XI. or XII., as the force which 

 determines the mean annual dii'ection of the wind is to that which deflects it in 

 the particular month to which the latter arrow relates ; and the length of the cor- 

 responding portion of the curve (increased threefold, because the curves are drawn 

 upon a less scale)' is proportional to the resultant of the two forces. Thus, for 

 example, at Hampden, in Maine, the force which determines the mean annual direc- 

 tion, the deflecting force in the month of January, and the resultant of the two, 

 are to each other as the numbers 33, 22, and 42; and by measuring the arrow. 

 No. 50, on Plate VIII., that for Januai-y on Plate XL, No. 25, and the first divi- 

 sion of the curve No. 25, on Plate VIIL, increased threefold, it will be seen that 

 their lengths are to each other in the ratio of these numbers. 



Now, if with the light of these explanations we examine Plate XL, we shall 

 notice that the arrows point with great uniformity toward the land in the warmer 

 months, and toward the sea in the colder. The cause is to be found in the differ- 

 ence of the temperature of the two. It is well known that the surface of large 

 bodies of water, and particularly the ocean, is much more uniform in its tempera- 

 ture throughout the year than that of land, and consequently must be colder in 

 summer and warmer in winter. Hence, we may account for the monsoon charac- 

 ter of the whuls on the opposite shores of the Atlantic, just as we do for the well 

 known phenomena of land and sea breezes on the sea-coast; the only difference 

 being that the former are on a more extensive scale. 



These views are confirmed, when we examine particular localities and sections of 

 country. The decrease in the curvature of the curves, as we recede from the sea- 

 shore, has been already adverted to, the examples mentioned being 29, 30, 32, and 

 40, on Plate VIIL, as compared with places near the coast. If we now look at the 

 same numbers on Plate XL, we shall see by the shortness of the arrows that the 

 deflecting forces, though conformable to the theory, are much less than at places 

 neai'er the sea.^ The absence of the monsoon character in the winds of Ohio is 

 probably to be ascribed to the fact that that State lies directly between the ocean 

 and the great lakes, so that the latter, being nearer, neutralize the influence of the 



* It would be more convenient for comparison if all could bo drawn on the same scale, i. e. if the curves 

 could be drawn upon a scale three times larger than they are; but they could not in that case be repre- 

 sented upon the maps without making the latter of unwieldy size. 



" No. 30 should properly be placed farther west, as the places whose results it represents are scattered 

 pretty uniformly over the southern half of the State of New York. 



