52 THE VOYAGE OF H.M.S. CHALLENGER. 



Australia. Such regions, and they are well marked, are found in all months and in all 

 oceans about lat. 30° to 40° N. and S., immediately to the westward of the continental 

 masses in these latitudes. The only exception to this is in the North Atlantic in January, 

 and the isobars of this part of the ocean for the months immediately following suggest 

 that this is a true exception. Lieut. Baillie's Isobaric and Current Charts of the Ocean 

 show in an instructive manner that the central spaces of these anticyclonic regions are 

 nearly always avoided by seamen, and therefore practically long known to them. It is 

 scarcely necessary to add that the prevailing winds blow out of them in all directions ; 

 and since these winds have the temperature of the upper regions whence they have 

 come increased only by the increasing pressure to which they are subjected as they 

 descend, their temperature often differs considerably from that of the surface of the 

 sea over which they blow. 



The lowest isobar, 28"90 inches, is found in the Antarctic regions to the east of 

 New Victoria. The observations of all the months show that there is a permanently 

 low pressure over these regions, lower than is to be found anywhere else on the globe. 

 On all the maps pressure is drawn to the isobar of 29 '30 inches, since observations 

 appear to warrant this ; but during the summer months of the southern hemisphere 

 lower isobars have been drawn for the portions of Antarctic regions for which observa- 

 tions have been furnished by the various expeditions which have been made into these 

 southern seas. 



The most wide-spread low pressure area is in tropical regions, where pressure, except 

 in the eastern half of the Pacific, falls below 29*85 inches. In this extensive region, 

 which covers about two-fifths of the whole surface of the globe, there are three areas 

 where pressure falls still lower. These are the north-west of Australia, Southern 

 Africa, and South America. A line drawn round the globe along the path of least 

 pressure of this zone separates the north and south " trades," indicating the belt or still 

 narrower zone towards which these great aerial currents blow. In the Atlantic and 

 eastern half of the Pacific, where the barometric gradient is well marked, these winds 

 are mapped out with equal distinctness ; but in the western part of the Pacific, where 

 the gradient is low and indistinctly marked, the direction of the prevailing winds is 

 irregular and obscure, and it is probable that increased observation will the more 

 strongly illustrate this remark. 



It will be observed that the path of least pressure lies north of the equator in the 

 Atlantic and Pacific Oceans. But in the Indian Ocean it is, at this season, south of it, 

 lying in a line from Seychelles to the north of Australia. In this restricted region the 

 winds are especially interesting as illustrating Buys Ballot's Law of the Wind in the 

 Southern Hemisphere. 



The next most important low-pressure system overspreads the northern part of 

 the Atlantic and regions adjoining, the lowest mean pressure being 29 - 50 inches from 



