METEOROLOGICAL CORRESPONDENCE. 407 



Anothei' peculiarity is an exemption from frosts in autumn. We almost in- 

 variably see notices of frosts in the southern States before we experience any- 

 thing of the kind here. 



Remarks. — The want of dew on Kelley's island, in lake Erie, may perhaps 

 be accounted for by the great amount of vapor which must exist in the atmo- 

 sphere over the great lakes, and which after the sun begins to decline is slightly 

 condensed into a thiti haziness, which prevents the radiation from the earth. 

 The exemption from early frost is perhaps partly due to the same cause and 

 partly to the equalizing effect of the large body of water on the temperature of 

 the air which is contiguous to it. 



From Conrad Mallinikroot, Augusta, Missouri. 



June, 1859. 



My twenty years' observations at this place seem to prove that the changes 

 of the wind follow the motion of the sun, viz : from east to south ; from south 

 to west ; from west to north ; from north to east, and so on. 



If it once springs back in the other direction it is of short duration, and soon 

 the main succession is restored. 



Remark.s. — This has been shown by Dove to be the normal motion of the 

 points of the weathercock in the northern latitude, and has been called the nor- 

 mal rotation of the wind. In places south of the equator the rotation takes 

 place in an opposite direction. 



The explanation of the phenomenon is not difficult when considered in rela- 

 tion to the different easterly velocity of different parts of the earth's surfiice as 

 it revolves on its axis. A cannon ball projected directly toward the north de- 

 scribes on the surface of the earth a line curving towards the east, on account 

 of the eastward motion which it received from the earth at the point of starting 

 being greater than that possessed by the several points of the earth's sui-face 

 which is passed over in the flight of the ball to northward. The longer the 

 motion of the ball continues the greater will be the curvature. For a similar 

 reason a ball shot directly south will describe a line curving to the west. Now, 

 suppose a wind produced by rarefaction of the air, or by any other cause, to 

 commence near an observer, and to blow directly from the south in a broad stream. 

 If it continues to blow, say for several hours, the particles of air which compose 

 the stream must start from points more and more remote, as would be the case 

 with the particles of water in a long trough of which the north end is suddenly 

 removed. Each particle of air in the stream must curve to the east, and the 

 greater the distance it has travelled the greater must be its easterly motion. 

 The particles of air which first strike the weathercock must turn the point of 

 the arrow very nearly south, while those which impinge against it after the 

 wind has blown for some time, having a greater easterly motion, must direct the 

 arrow to the west of south, the westerly direction increasing as long as the wind 

 continues to blow from more southern parallels of latitude. For a similar 

 reason a wind commencing to blow from the north must turn the point of the 

 arrow more and more to the east the longer it continues to flow southward. A 

 similar result, but differing in degree, must be produced from whatever inter- 

 mediate point the wind commences to blow, providing the current continues to 

 flow in a broad stream for a few hours. 



Hence, as a consecjuence of the rotation of the earth on its axis, the wind, 

 at a given place in its normal flow, must cause the point of the arrow to move 

 in the direction of the apparent motion of the sun. 



When the arrow is observed to turn in an opposite direction we may be sure 



