86 



RECORD AND DISCUSSION OF ATMOSPHERIC PRESSURE. 



and aneroid barometers, the reduction of the former to the temperature 32° is 

 appHed by Mr. Sonntag, astronomer to the expedition, for the case of a brass con- 

 nection between the cistern and scale. Table No. XVII. Series C, p. 63, in 

 Guyot's Meteorological and Physical Tables, edition of 1858, prepared for the 

 Smithsonian Institution, answers this case, and has been adopted by me for the 

 temperature reductions. This reduction, in general, is not very great, as may be 

 seen on p. 425, Appendix No. XII. of the second volume of the Narrative, where 

 the mean readings of the attached thermometer, fur the year 185i, is given 

 as +40°.3. 



Reductkm of the Aneroid to the Mercurial Barometer. — A number of comparisons 

 of the two instruments, for the purpose of reducing the readings of the first to 

 their equivalent values by the second instrument, wei'e made, as stated above. 

 The diflerences in the indications of the mercurial and aneroid barometers, or 

 M — A, may be supposed to be proportional to the variations in pressure and tem- 

 perature, and may be represented by the equation 



B = A -\- c + m {A — mean pressure) + n {T — mean temperature), 

 c being an index error. The quantities c, m, n, are to be determined from the 

 following comparisons. In the absence of any better information, the attached 

 thermometers were supposed free of index error. The comparisons were made by 

 Mr. Sonntag. 



