DESCEIPTION OP MAPS ATs^D DIAGEAMS, 



The direction of tlie wind on the maps, Plates 1 to 13, is indicated by arrows. For example, 

 to indicate a N. .wind, the head of the arrow is turoed towards the south, and the tail towards the 

 north. The direction indicated is not that of the prevailing wind, but the mean direction, the manner 

 of calculation of which was explained in the beginning of this work. The length of the stem of the 

 arrow, exclusive of its barb, is proportionate to the ratio of resultant, the greatest length being when 

 the ratio is equal to 100, or when all winds come from the same direction. These maps were origi- 

 nally all drawn to a scale, in which one hundred per cent., as found in the tables, was intended to 

 be represented by an arrow an inch in length ; but, by the process of engraving adopted, it was 

 found practicable to diminish the size of the maps somewhat, so that 100 per cent, equals two-thirds 

 of an inch; for instance, on Plate 1, Zone 10, serial number 196, representing Eastern Pennsyl- 

 vania, the arrow is 20 one-hundredths of an inch in length, corresponding to the tabular percentage 

 .30 given on page 320. The more equally the winds are distributed around the horizon the smaller 

 is the ratio, and also the shorter the arrow on the map. Where it is very small it indicates that there 

 is no really prevailing wind. This is generally found on the boundaries of two systems of winds. 



PLATE 1. 



Annual Direction of Upper and Lower Current in the United States. 



The mean direction of the wind, as observed by the wind-vane, is indicated by full arrows, and the 

 direction of the motion of clouds by broken arrows. It will be seen that they very nearly coincide 

 in nearly all regions of the United States. Generally the upper current is more purely west in all 

 the regions east of the Mississippi, while the lower current has a more W. S. W. direction between 

 the Mississippi and Apallachian Chain, as well as in the Southern Atlantic States, while in New 

 England the winds are rather W. N. W. Near the Gulf of Mexico the arrows have a very different 

 direction, but it will be seen that the arrows are very small, thus indicating an undecided prevalence 

 of any wind. In some parts of Texas, also, the upper and lower current seem to come from different 

 directions. In this map, as well as in the others, the figures relate to the serial number in the zone, 

 and, by reference to the Numerical Index to Stations, given on pages 52 to 66, it is easy to find the 

 name of the place indicated by each figure. 



PLATE 2. 



Mean Annual Direction in the Arctic Regions. 



It will be noticed that the mean direction of the wind is from the north in Greenland and Arctic 

 America, and that the arrows are long, thus indicating very prevailing winds. On the northern 

 coasts of Europe and Asia the winds are from the south, while Bear Island, between Norway and 

 Spitzbergen. as well as Iceland, have prevailing easterly (polar) winds. In this map a dotted line is 

 traced and called "Southern limit of polar system." This is the same boundary as that traced by 



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