190 SCIENCE IN SHOET CHAPTERS. 



invented the art of writing, and we honor his memory accord- 

 ingly. But he ventured no further than teaching human 

 beings to write. Modern meteorologists have gone much 

 further ; they have taught the winds and the rains and the 

 subtle heavings of the invisible air to keep their own diaries, 

 to write their own histories on paper that is laid before them, 

 with pencils that are placed in their tieshless, boneless, and 

 shapeless fingers. This achievement is wrought by compara- 

 tively simple means. The paper is wound upon an upright 

 drum or cylinder, and this cylinder is made to revolve by 

 clock-work, in such a mannner that a certain breadth travels on 

 during the twenty-four hours. This breadth of paper is divided 

 by vertical lines into twenty-four parts, each of which passes 

 onward in* one hour. Connected with the barometer is a 

 pencil which, by means of a spring, pi esses lightly upon the 

 revolving sheet, and this pencil, while thus pressing, rises and 

 falls with the mercnry. It is obvious that, in this manner, a 

 line will be drawn as the paper moves. If the mercury is 

 stationary, the line will be horizontal only indicating the 

 movement of the drum ; if the mercury tails, the line will 

 slope downward ; if it rises, it will incline upward. By rul- 

 ing horizontal lines upon the paper, representing 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 windward 

 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 pro- 

 portion to this pressure. This movement of the spring is 

 transmitted mechanically to another pencil like the above 

 described, working against the same drum ; thus another his- 

 tory is 'written on the same paper the horizontal lines now 

 representing fractions of pounds of pressure, instead of frac- 

 tions of inches of mercury. 



