ATMOSPHERIC OCEAN TIDES. 147 



sented by regions of high pressure, and low tides 

 by areas of opposite conditions. There are two of 

 these great regions of high atmospheric pressure, 

 one to the north, and the other to the south of the 

 equator, both of which extend precisely as in the case 

 of our Climatic Zones, as broad belts completely round 

 the earth. The highest of these tides is usually 

 located during winter in the centre of a great continent 

 notably in that of Central Asia ; and in the summer 

 time they shift to the surface of the ocean, which 

 then becomes the area where the maximum atmo- 

 spheric pressure prevails, especially in the Southern 

 hemispheres, (where the location is much less subject 

 to change than in the Northern, because of the 

 enormously greater predominance of ocean there). The 

 most common location for the area of maximum 

 pressure at this time of year may be generally roughly 

 said to be about latitude 30. * This, however, is a 

 general indication of an approximate nature only. 



The weight of the atmosphere is generally measured 

 by the barometer, which rises with increasing pressure ; 

 whereas, as a weather-glass, the barometer only 

 inferentially indicates what the weather is likely to be. 

 Dry air is heavier than damp air: therefore when 

 the barometer goes up, we infer that it will be fine 

 weather, because the air is comparatively dry. Sudden 

 ups and downs in the glass, however, are probably 

 caused by the transit of a great atmospheric billow 

 in the zenith; as this passes above us, the increased 

 weight of the air causes the mercury to rise, and when 



* The British Admiralty has published a series of charts, which show 

 the mean pressure of the atmosphere over the Ocean, at the different 

 seasons. 



