Page:Physical Geography of the Sea and its Meteorology.djvu/202

176 Number of Observations and Mean Height of the Barometer between the Parallels of 78° 37’ N. and 74° S.

away from the 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 comparison for the barometer at sea.—Admiral Fitzroy has also reduced from the abstract logs in the Meteorological Department of the Board of Trade in London a great number of barometrical observations. He has discovered the near the parallel of 5° N. in the Atlantic Ocean the pressure of the atmosphere is so uniform as to afford navigators a natural standard by which, out there at sea, they may, as they pass to and 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