THE BAROMETER AXD THE WEATHER. 151 



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 convex faces foremost; and they 

 will move with one third of the actual velocity of the wind". 

 By a simple clock-work arrangement, these arms move an- 

 other pencil, in such a manner that it strikes the paper 

 hammer-fashion every time the wind has completed a jour- 

 ney 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 pres- 

 sure do duty 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 an open mouth of measured area say 100 square 

 inches. This receives the rain that falls. By means of a 

 pipe the water is conveyed to a vessel having a surface of 

 say one square inch. By this arrangement, when sufficient 

 rain has fallen to cover the surface of the earth to the depth 

 of one hundredth of an inch, the little vessel below will 

 contain water one inch in depth. By balancing this vessel 

 at the end of a long arm, it is made to preponderate grad- 

 ually as the weight of water it receives increases, and finally, 

 when filled, it tips over altogether, empties itself, and then 

 rises to its starting place in equilibrium. To the other end 

 of this arm a pencil is attached, which inscribes all these 



