3% (Proceedings of the 



JANUARY 8, 1876. 



The Association met pursuant to previous adjournment, 

 and was called to order by the President, Dr. Cocker. 



The minutes of the meeting of January ist were read and 

 approved. 



Propositions for membership of A. P. Wood and Chas. 

 Tripp were received and referred to the Board of Censors. 



Prof. Pettee was here introduced, and read a paper on 

 " Barometric Measurements of Altitudes." 



The object of this paper was to discuss questions relating to 

 the degree of accuracy attainable in measuring the height of 

 mountains by means of the mercurial barometer ; and to exhibit 

 certain interesting results obtained by Mr. Pettee when engaged 

 as assistant upon the State Geological Survey of California, 

 under the direction of Prof. J. D. Whitney. 



The detailed account of the experiments instituted in Cali- 

 fornia, and of the manner of carrying on the work, may be 

 found in the publications of that survey. 



For a period of three years, barometric and thermometric 

 observations were taken three times a day at three different sta- 

 tions, the altitudes of which above the sea-level were known 

 from the spirit-level surveys of the Central Pacific Railroad. 

 These observations were taken as the data from which to calcu- 

 late in the usual way the differences of altitude between the re- 

 spective stations. When the calculated differences of altitude 

 were compared with the known differences, certain remarkable 

 discrepancies became evident. The calculated differences were 

 always higher in summer than in winter, higher at noon that at 

 morning or night, and in some cases higher, in others lower, 

 than the true differences. The practical benefits to be derived 

 from such an investigation is the guide it affords to the explorer 

 in selecting the time of day or the season of the year in which 



