18 INSTIILX'TIONS TO MARINE METEOROLOGICAL OBSERVERS 



shells are made into a vertical coluinu by screwing one to another. 

 The lower base of the column being fixed, the upper end rises and 

 falls with ev-ery variation in the atmospheric pressure, by a quantity 

 which is the sum of the displacements of the elementary shells. 



The compensation for temperature is accomplished by leaving a 

 sufficient quantity of air in one of the shells, ascertained by experi- 

 ment when the instrument is made, so that with a rise of temperature 

 the tendency of the barometer to register too low on account of the 

 weakening of the springs, and the expansion of the levers and other 

 parts is counteracted by the increased pressure of the air in the shell. 

 However, the instrument should be kept at a uniform temperature 

 as far as possible. 



Figure 9. — Richard's aneroid barograph. 



Reading of aneroid harometers. — Tliese instruments are usually 

 graduated to every two-hundredths of an inch, and reading of them 

 to hundredths is, therefore, easy. Care, however, should be taken not 

 to confuse tlie tenths divisions with those for hundredths. Figure 

 10 is a reproduction of a portion of the dial of a common type of 

 aneroid barometer, to which has been affixed a series of imaginary 

 positions of the needle marked "«", "'b''\ ""<?", etc. The following are 

 the correct readings corresponding to these positions : 



(a) 28.75 inches. (^/) 29.56 inches. 



(h) 29.06 inches. (e) 30.00 inches. 



[c) 29.20 inches. (/) 30.10 inches. 



Defects in aneroids. — There are many sources of mechanical de- 

 fects in aneroid barometers. This is especially true of those of in- 

 expensive manufacture, and of those which do not have the dealer's 

 guaranty of good performance. Aside from the defects induced by 

 long usage and stress of weather on shipboard, several may be men- 



