682 EEMAEKS ON THE BAROMETEB. 



hermetically sealed, metallic box, from which almost all the air Las been 

 withdrawn, and which is kept from collapsing by a spring. The top of 

 the box is corrugated. When the atmospheric pressure rises above the 

 amotint which was recorded when the instrument was made, the top is 

 forced inwards ; and vice versa, when the pressure falls below that amount, 

 the top is pushed outwards by the spring. These motions are transferred 

 by a system of levers and springs to a pointer, which moves on a dial like 

 that of a wheel-barometer. 



It is at once evident that the instrument must be graduated experimen- 

 tally, as it cannot measure pressure absolutely, but afifords indications 

 relatively to a mercurial Barometer, its sensibility depending, inter alia, 

 on the quality of the metal of which the box is made. 



Aneroids are very sensitive, but unfortunately they do not preserve their 

 accuracy. If a table of corrections be determined for an Aneroid, it will 

 be found that after a time the latter has undergone some change, and 

 that the values of the corrections will require alteration, so that re- 

 comparison with a Standard Barometer will be necessary (see page 108). 

 In every case of such comparison the readings of the mercurial Barometer 

 should be reduced to 32°. 



A most serious objection to the scientific utility of these instruments is 

 their liability to injury, owing to rust, or to the alteration of force in the 

 springs used in their construction. However, for the reasons above stated, 

 the Aneroid is especially suitable for fishermen and pilots, or seafaring 

 persons employed in boats or small coasting vessels, in which there is not 

 space to suspend a Barometer. 



For further information, on the practical use of the Barometer, reference 

 should be made to the Section on "The Motions and Pressure of the 

 Atmosphere," pages 108 — 124. 



