INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS IQ 



tioned. For instance, springs and linkage may be too tight and 

 sluggish, so that i-esponse to pressure changes, great or small, is slow 

 or incomplete. In other words, the hand sticks, and if it finally suc- 

 ceeds in pointing to a very low pressure, it may fail to return prop- 

 erly to indicate the following much higher pressure. Likewise, it 

 may fail, except partly, to indicate any change in pressure. 



Other aneroids, due to poor construction of adjusting parts, are 

 so erratic in action that, for meteorological purposes, they are prac- 

 tically worthless. 



Then there is the "creeping" error, which troubles many aneroids, 

 and produces a gradual increase in the correction until it assumes 

 large proportions. Creeping may be due to one or more causes. 

 There may be porosity in the metal box inside the instrument which 

 admits minute quantities of air into the vacuum space and thus 

 gradually destroys the power to respond to the pressure of the at- 



FiGDRE 10. — Illustrating six different readings of the aneroid barometer. 



mosphere ; or, the heavy spring which opposes the air pressure may 

 be crystallizing slowly and thus more and more preventing the 

 vacuum element from responding to the outer-air pressure. An 

 aneroid with creeping trouble may become worthless in time, but 

 until that stage occurs, if the change in error is very slow, frequent 

 comparisons will keep pace with it, and the instrument may be 

 serviceable for a considerable period. 



Some aneroids do not correctly register considerable changes in 

 pressure, though they may continue reliable under ordinary pres- 

 sures. Let the pressure fall to a low value, as in a deep cyclonic 

 storm, such instruments either fail to keep pace with the change, or 

 come to a dead stop at some point while the pressure is still f allin.o;. 



Oo77ipensation for teiivperature. — Another defect, and a very com- 

 mon and important one, is entire lack, or insufficiency, of a tem- 

 perature-compensating device in the instrument. 



All aneroid barometers that are properly manufactured with the 

 purpose of giving correct pressure readings, are fitted with a means 

 in the mechanism which automatically compensates for the effects 

 of temperature changes upon the dimensions and elasticity of the 



