154 



GENERAL SCIENCE 



(b) The Aneroid Barometer. Since the mercurial 

 barometer is long and inconvenient to carry, geologists, 

 surveyors, and mountain climbers commonly use the 

 instrument called the aneroid barometer. It consists 

 essentially of an air-tight cylindrical box, the top of 

 which is a metallic diaphragm which bends slightly under 



the influence of a change 

 in the atmospheric pres- 

 sure. If the air pressure 

 increases, the diaphragm is 

 pushed slightly inward; if 

 the air pressure decreases, 

 the diaphragm springs out- 

 ward. This motion of the 

 diaphragm is multiplied 

 by a delicate system of 

 levers, and is communi- 

 cated to a hand which 



AN ANEROID BAROMETER 



Qver 



readings are made to correspond to the readings of a 

 mercury barometer. There are aneroid barometers made 

 so sensitive that they will indicate a change in pressure 

 when they are moved from a table to the floor. The 

 weather conditions are printed on the face of these ba- 

 rometers because the barometric readings and weather 

 conditions usually agree. 



Some aneroid barometers have two hands, one fixed 

 and the other movable. The fixed hand is set over the 

 movable one at a certain hour and in reading the instru- 

 ment one can tell whether the air pressure has increased 

 or decreased since that hour, for the movable hand will 

 change its position as the atmospheric pressure changes. 



Some aneroid barometers are made in the shape of a 



