328 Voyage of the Novara. 



commencing upon a definite plan, might obtain results such 

 as might hand down the scientific renown of our age and 

 century to all future generations. 



While sailing in these southern latitudes, the Commodore 

 hit upon the idea of ascertaining the increase of gravity as 

 the poles were approached, by the comparison of simultaneous 

 observations taken with the mercurial and Aneroid barome- 

 ters. Both instruments, in fact, gave a regular rule for calcu- 

 lating the weight of the atmosphere at the points of observa- 

 tion, ^Yit\\ this single difference, that the ordinary barometer 

 gives the weight by the pressure of the aii^ upon a column of 

 mercury, representing the weight of a similar column of air ; 

 while in the Aneroid barometer, the weight of the atmo- 

 sphere is measured by an exhausted receiver, which, in resist- 

 ing this pressure, indicates the amount by the tension of a 

 spring. 



The indications of the Aneroid are moreover independent 

 of the influence of universal gravity and the disturbing con- 

 ditions it introduces into the instrument, to which the column 

 of mercury is of course subject. Assuming, for example, that 

 tlie ordinary barometer and the Aneroid gave the same read- 

 ings, the similarity will no longer exist at a given distance 

 from the Equator — the Aneroid, owing to the elimination of 

 the disturbing element of gravity, indicating an increased 

 pressure of the column of air, whereas the ordinary barometer 

 will continue to indicate the same pressure as at the Equator. 

 The difference between the two readings will, however, be 



