232 TENTH ANNUAL REPORT OF 



[As a flat vane is always in a neutral line, a more accurate and 

 sensitive one is made by fastening two plates together at an angle of 

 about ten degrees, forming a long wedge. 



The longer the vane, the shorter the pulsations, and the steadier the 

 action will be. For a small sized vane, it may be ten or twelve inches 

 wide, and four feet long.] 



Observation. — The observation of this instrument demands some 

 care. In winds of considerable strength the vane is never at rest, or 

 fixed in the same direction ; it oscillates incessantly, and its oscilla- 

 tions increase in amijlitude with certain w^inds, and with the violence 

 of the wind. We must then note the mean direction between the ex- 

 tremes. When the wind is very feeble, perhaps it may not have 

 sufficient force to set the vane in motion ; in this case, as when the 

 air is calm, great mistakes might be made by registering the direc- 

 tion marked by the index ; for its position indicates, not the direction of 

 the existing wind, but that of the last wind that had the power to set 

 the instrument in motion. When the index is immovable, and there is 

 no oscillation, we must give up its indications, and refer to the move- 

 ment of light bodies, as that of the leaves of trees and the smoke of 

 chimneys, to determine the direction of these feeble currents of air. 

 During the night the direction of the wind may be easily ascertained 

 by raing the hand in the air, with one finger wet. The least motion 

 in the air increases evaporation, and a sensation of cold is experienced 

 on the side of the finger turned towards the wind. 



The direction of the wind must be noted, following the eight prin- 

 cipal points of the compass — north, northeast, east, southeast, south, 

 southwest, west, and northwest. For the additional observations 

 during storms, the degrees may be indicated, in order to follow more 

 exactly the rotation of the wind, or at least sixteen points of the com- 

 pass, viz : N. NNE. NE. ENE. E. SE. ESE. SSE. S. SSW. SW. 

 WSV\^. W. WNW. NW. NNW. 



The lower, or surface wind, often has a difierent direction from that 

 which prevails in the ujoper regions of the atmosphere, and this is 

 generally the case when the wind turns, and the weather is going to 

 change, also during storms and great atmospheric movements. The 

 direction, then, of the lower and the higher layers of clouds must be 

 separately noted in the several columns of the journal reserved for this 

 purpose. If the direction is the same in the whole extent of the at- 

 mosphere, the same letters will be marked in the three columns. If 

 the absence of clouds does not permit us to judge how the wind is 

 above, a dash must be substituted for the letter, indicating that the 

 observation has been made. A blank always signifies an observation 

 omitted. 



To avoid an error in the estimate of the direction of the clouds, it 

 will be well to observe their course between two fixed points, as a 

 window frame, the fixed lines of which will facilitate the observation. 

 Another very convenient method is to place a small mirror horizon- 

 tally, with lines traced on it indicating the points" of the compass ; 

 the image of the clouds passing over these will indicate their direc- 

 tion. 



