4 THE AIR 



although in some points the methods, necessarily 

 restricted to the convenient use of a mercantile marine 

 officer without interfering with his more urgent duties, 

 are capable of considerable expansion in the hands of 

 a scientific student. 



In the United States the Weather Bureau carries on 

 similar researches, and the German Naval Observatory 

 (Deutsche Seewarte) in Hamburg is equally active in 

 the work. Both the British Meteorological Office and 

 the United States Weather Bureau issue in advance 

 Monthly Pilot Charts of the great oceans, on which the 

 average meteorological conditions for the month are 

 laid down. 



The introduction by Maury of sailing directions, 

 based on a knowledge of prevailing winds and currents, 

 was of great profit to shipowners by shortening 

 voyages, and stimulated the interest of shipmasters, 

 by whom most of our available information of maritime 

 meteorology was collected in the old sailing-ships. 

 The comparative unimportance of weather conditions 

 in the voyages of modern steamships has led to the 

 partial neglect of weather observations by sailors, 

 though the officers of many vessels keep and transmit 

 to the proper authorities excellent meteorological logs. 



Brief Sketch of Atmospheric Circulation and 

 Temperature. 



The sun shining on the atmosphere, particularly 

 where the ground below is a good absorbent and 

 radiator of heat, warms it, and causes the air to expand, 

 making it lighter, and so reducing its pressure on the 

 surface. The colder and heavier air of neighbouring 

 regions on which the sun is not shining (either because 

 it is night or because of clouds obscuring its rays) is 



