THE BAROMETER AXD THE WEATHER. 



191 



It has been found that if a semi-globular cup of thin metal 

 is exposed to the wind, the pressure upon the round or convex 

 side of the hemisphere is equal to two thirds of that upon the 

 hollow or concave side. By placing four such cups upon 

 cross-arms, and the arms on a pivot, the wind, from whatever 

 quarter it may come, will always blow them round with their con- 

 vex faces foremost ; and they will move with one third of the 

 actual velocity of the wind. By a simple clock-work arrange- 

 ment, these arms move another pencil, in such a manner that 

 it strikes the paper hammer- fashion every time the wind has 

 completed a journey of one mile, or other given distance ; and 

 thus a series of dots upon the revolving paper records the 

 velocity of the wind according to their distances apart. As 

 the pressure of the wind is governed by two factors viz., the 



density and velocity of the moving air the relations between 

 the barometer curve, the pressure curve and the velocity dots 

 are very interesting. 



The direction of the wind is written by a pencil fixed to a 

 quick worm a screw thread upon the axis of the vane. As 

 the vane^turns round N., E., S., or W. it screws the pencil 

 up or down, and thus the horizontal lines first described as 

 registering tenths of inches of barometric pressure do dutv as 

 showing the points of the compass from which the wind is 

 blowing ; and, by reference to the zigzag line drawn by this 

 pencil of the wind, its direction at any particular time of day 

 may be ascertained as certified by its own sign manual. 



The wind-gauge is called an anemometer. Connected with 

 this is the pluviometer, or rain-gauge an upright vessel with 



