WIND-VANE. 25 



If the dial is expose i to the open air, it must be protected 

 against the snow and ''ce, which would impede the play of the 

 pivot and of the index. A small ring of wood placed around 

 the pole, under oue of the friction rollers, will prevent the wind- 

 vane from being raised, and the pivot from being displaced dur- 

 ing the most violent winds. 



[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. Thus, 



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 oscillations increase in amplitude with certain winds, and 

 with the violence of the wind. We must then note the mean 

 direction between the extremes. When the wind is very feeble, 

 perhaps it may not have sufficient force to set the vane in "mo- 

 tion ; in this case, as when the air is calm, great mistakes might 

 be made by registering the direction 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 oscilla- 

 tion, we must give up its indications, and refer to the movement 

 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 raising 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 

 principal points of the compass — north, northeast, east, south- 

 east, 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 



