150 



SCIENCE IN SHORT CHAPTERS. 



upwards. By ruling horizontal lines upon the paper, rep- 

 resenting inches, tenths, and smaller fractions, if desired, 

 the whole history of the barometrical movements will be 

 graphically recorded by the waving or zigzag lines thus 

 drawn by the atmosphere itself. 



The subjoined copy of the Daily Telegraph Barometer 

 Chart represents, on a small scale, a four days' history of 

 barometrical movements: 



The large figures at the side (29 and 30) represent 

 inches; the smaller figures tenths of inches. 



The pressure of the wind is similarly pictured by means 

 of a large vane which turns with the wind, and to the wind- 

 ward face of which a flat board or plate of metal, one foot 



square, is attached perpendicularly. As the wind strikes 

 this it presses against it with a force corresponding to a 

 certain number of pounds, ounces, and fractions of an 

 ounce. A spring like that of an ordinary spring letter- 

 balance is compressed in proportion to this pressure. This 

 movement of the spring is transmitted mechanically to an- 

 other pencil like the above described, working against the 

 same drum ; thus another history is written on the same 

 paper the horizontal lines now representing fractions of 

 pounds of pressure, instead of fractions of inches of mer- 

 cury. 



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

 metal is exposed to the wind, the pressure upon the round 



