776 The Outline of Science 



aneroid, which consists essentially of a small thin circular jorru- 

 gated metal box from the interior of which the air has heen 

 exhausted, thereby forming a vacuum within the box. The air- 

 pressure acts upon the sides of the box, causing them to yield or 

 to recover against the action of a spring, and by a system of 

 levers these movements are communicated to a hand which moves 

 over a dial upon which a scale is engraved. The aneroid is a con- 

 venient and portable instrument, but is much less sensitive and 

 accurate than the mercury barometer. Its chief application in 

 meteorology is in the form of the familiar "barograph," an instru- 

 ment wherein eight of these small vacuum boxes are superposed, 

 and their combined movement is magnified by a system of levers 

 and transmitted to a pen, which then traces a record of the 

 movement upon a paper chart mounted upon a drum actuated by 

 clockwork and making usually one revolution in a week. The 

 trace thus obtained is a very useful and reliable guide to the 

 pressure changes in progress. 



The temperature of the air is recorded by means of ther- 

 mometers, instruments whose appearance is very familiar to 

 everyone. A thermometer consists of a glass tube having an ex- 

 tremely narrow bore, which is enlarged at one end into a bulb, 

 either spherical or cylindrical in shape. The bulb is filled with 

 mercury, which likewise fills part of the narrow bore in the stem of 

 the thermometer. If the temperature increases the mercury in the 

 bulb expands and the column in the bore rises, while a cooling of 

 the air causes contraction of the mercury and a decrease in the 

 height of the column. 



The Stevenson Screen 



Meteorological stations are equipped with a set of ther- 

 mometers for recording various facts regarding the temperature. 

 The instruments are housed in a box of special design known as 

 a "Stevenson Screen." This is usually about 20 inches wide from 

 east to west, 13 inches deep in the north-south direction, and 14 



