THE SMITHSONIAN INSTITUTION. 231 



sured as so miicli rain. When the snow is adhesive the use of the tin 

 plate will not be necessary. 



From measurements of this kind^ repeated in several places when the 

 depth of the snow is unequal, an average quantity may be obtained. 



As a general average, it will be found that about ten inches of snow 

 will make one of water.] 



WIND-VANE. 



Placing. — The ivlnd-vane should be set in a place as free and open 

 as possible, away from every obstacle, and especially from high build- 

 ings. It should exceed in elevation, by at least eight or ten feet, the 

 neighboring objects. To facilitate observations at night, the follow- 

 ing 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 win- 

 dow. The axis terminates by a steel pivot turning freely on a 

 cast-iron plate. This plate supports a dial divided into degrees, be- 

 sides 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 difi'erent 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. 



If the dial is exposed to the open air, it must be protected against 

 the snow and ice, which would impede the play of the pivot and of 

 the index. A small ring of wood placed around the pole, under one 

 of the friction rollers, will prevent the wind-vane from being raised, 

 and the pivot from being displaced during the most violent winds. 



