176 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



Niimher of Ohservafions and iV]ea» Height of the Barometer between the Paralleb 



of 78^ 37' N. and 74^ S* 



(1) From 50° N. to 36° S. the observations are the mean of 83,334 taken from " Maandelijksche 

 Zeilaanwijzingen van Java naar Let Kanaal Koninklijk Nederlandsch Meteorologisch liisiituut, 

 1859." 



(2) Greenwich ; mean of 4 years' observations. 



(3) St. Petersburg; mean of 10 years' observations. 



(*) ])r. Kane"; 12,000 observations (mean of 17 months' observations). 



(5) Hobart Town ; mean of 10 years' observations. 



(6) Sir J. C. Ross ; " Erebus and Terror." 



away from tlie austral regions, just as the vapour that is formed 

 in the real steam-boiler expels the air from it. This difference 

 of atmosphere over the two halves of the globe, as indicated by 

 the barometer, is very suggestive. 



364. A standard of comjparison for tJie harometei- at sea. — Admiral 

 Pitzroy has also reduced from the abstract logs in the Meteoro- 

 logical Department of the Board of Trade in London a great 

 number of barometrical observations. He has discovered that 

 near the parallel of 5° N. in the Atlantic Ocean the j)ressure of 

 the atmosphere is so uniform as to atford navigators a natural 

 standard by which, out there at sea, they may, as they pass to 

 iind fro, compare their barometers. This pressure is said to be 

 so uniform, that after allowing for the six-hourly fluctuations, 

 the mariner may detect any error in his barometer amounting to 

 the two or three thousandth part of an inch. 



365. South-east trade-winds having no moisture traced over into 



* Below the parallels of 50^ N. and 36° S. the observations are reduced to 

 the temp, of 32"^ Fahr. 



