194 STATE HORTICULTURAL SOCIETY. 



represented by the direction of the arrows in the diagram 

 (1). If this system has a progressive motion the place 

 over which it passes has a succession of winds from dif- 

 ferent directions. If the direction is east, a point in the ^ ''^__^ +& 

 path of the centre, as (b) in the diagram, will have first 

 a west, then an east wind ; a point to the north of the 

 path of the centre, (a) of the diagram, will have first a 

 southwest, then a south, then a south-east wind ; and a * +c 



point to the south of the path of the centre (c), will have 



a wind which begins at the northwest and swings through north to northeast. 

 By varying the direction of the system and the position of the point passed 

 over, we may get any succession of from two to five of the eight principal 

 directions. 



In the centripetal system we have a whirlwind of greater or less dimensions. 

 It has often been suspected that, in some cases, at least, the currents here 

 flow directly in toward the center, but no positively convincing case has been 

 published so far as known to the writer. In every case satisfactorily investi- 

 gated the wind flows in toward the center in a spiral some- 

 thing, as shown in diagram (2). As this system passes 

 over a place there is also a succession of winds, but the 

 order in which they follow is different from that of the. 

 centrifugal system. If in this case the progressive mo- 

 tion is east, and a point (b) lies in the path of the center, 

 its winds are first south, then north. If it lies to the 

 north of the center, as (a), the wind is first southeast 

 and swings through east to northeast; if the place is 2. 



south of the center, as (c), the wind begins at southwest and swings through 

 west to northwest. Similar successions may be found for the other progressive 

 directions and positions of the path of the center. 



We shall now classify the winds of Michigan as to their origin and range. 

 From this point of view they fall naturally into local, cyclonic, and general. 

 The local winds are of local origin, and limited range. The origin of the 

 cyclonic winds is not known, but they undoubtedly originate in conditions 

 which prevail at the same time over an extensive region, but are not gen- 

 eral. In range they pass sometimes nearly around the earth, but they exercise 

 no influence on the winds outside of their system. The general winds are 

 part of the interchange of air between equator and poles and are due to the 

 greater heat at the equator. 



I. LOCAL WINDS. 



a — Zephyrs. These are light winds of local origin and very small extent. 

 The region controlled by them at any given time may be a few rods or possibly 

 a few miles square. They are of brief duration, and the zephyr which dies 

 out now may be immediately replaced by a breeze from a different direction. 

 Zephyrs are parallel or centripetal and possibly sometimes centrifugal. 



b — Ascending vertical currents. They would not be felt by us at the earth's 

 surface, as that is the place of origin. They are due to the unequal heating 

 of the air near the ground. The warmer air is lighter and ascends, but we 

 see nothing of it until its altitude is so great that the cold of the upper 

 regions condenses its vapor in the form of white mountainous clouds called 

 cumuli. These winds occur in summer and are doubtless in a parallel system. 



