24: WIND-VANE. 



WIND-VANE. 



Placing. The wind-vane should be set in a place as free and 

 open as possible, away from every obstacle, and especially from 

 high buildings. It should exceed in elevation, by at least eight 

 or ten feet, the neighboring objects. To facilitate observations 

 at night, the following arrangement may be adopted : 



The wind-vane is composed of a leaf of zinc about three feet 

 in length, in the form of a butterfly's wing, exactly counter- 

 balanced by a leaden ball. It is carried upon a cylindrical axis 

 of pine wood, or of any other light and strong material, two 

 inches in diameter, which, if possible, passes down through the 

 roof into the observer's chamber, otherwise along the exterior 

 wall of the building to a window. The axis terminates by a 

 steel pivot turning freely on a cast-iron plate. This plate sup- 

 ports a dial divided into degrees, besides indicating the eight 

 principal points of the compass. The axis carries an index 

 placed in the same plane as the feather of the wind-vane, which 

 enables us to read upon the dial, as well by night as by day, the 

 direction of the wind. The whole rests on a strong wooden 

 shelf, firmly fastened to the window by supports. Above, the 

 rod is firmly fixed to a strong upright staff, or, better, on the 

 roof, with strong braces, by means of a piece of wood containing 

 friction rollers, which allow the shaft to turn freely and without 

 effort. Similar pieces with friction rollers, placed at different 

 distances along the wall, keep the axis vertical. 



Great care must be taken to secure the perfect verticality of 

 the shaft, and to this end it is necessary to fix it by a plumb-line 

 in two different planes cutting each other at right angles. The 

 index at the foot of the rod should be placed on the same side 

 with the point of the wind-vane, and in the same plane as the 

 feather. The pivot should turn very freely in the hole that 

 receives it, and into which a drop of oil should be poured. 



Finally, we must carefully adjust the points of the dial, which 

 is supported with the iron plate, upon a board fastened upon a 

 shelf by means of a strong screw. In making this adjustment 

 by means of a compass, the magnetic variation of the locality 

 must be taken into account ; each observer should have the line 

 of the true north traced on his window. 



