1872.] 



'Shearwater' Scientific Researches. 



609 



and the South Atlantic respectively (see Plate VII.) serves to show (as was 

 well pointed out in the * Onderzoekingen met den Zee-thermometer' pub- 

 lished in 1 86 1 by the Royal Meteorological Institute of the Netherlands) that, 

 while the North Atlantic is as a whole warmer than the South, its Tempera- 

 ture has also a much wider Annual range : for the corresponding Isotherms 

 lie at a greater distance from the Equator on its Northern than they do on 

 its Southern side ; and the central line of the belt of warmest water does 

 not coincide with the Equator, but lies for the most part on its Northern 

 side ; so that the Thermic Equator, or line of maximum mean Temperature, 

 falls some degrees to the north of the Geographical Equator. The line 

 which divides the two Hemispheres climatologically of course shifts its 

 position according to the seasons ; but all these changes of situation take 

 place to the north of the Equator, except in February and March, when 

 this line has its most southerly position, and crosses the Equator in a few 

 places. Again, comparing points at which the Isotherms cross the Meri- 

 dian of 30° W. Long, in the North Atlantic, with the points at which they 

 cross the Meridian of 20° W. Long, in the South Atlantic, — these two me- 

 ridians being, as it were, the axes of their respective basins, — we observe 

 that in the North Atlantic the distance between the Summer and the 

 Winter Isotherms of 60° is nearly 15° of Latitude, whilst in the South 

 Atlantic the distance between those two Isotherms is only about 7 J° ; and 

 that a like difference shows itself between the seasonal positions of the Iso- 

 therms of 55° and 65° in the Northern and Southern Oceans respectively. 

 The great regularity of the directions and distances of the Isotherms 

 beyond the parallel of 30° S. Lat. in the South Atlantic, also, is very 

 striking when contrasted with the irregularity, both in course and distance, 

 which marks those of the North Atlantic. The Annual Range is observed 

 to diminish in the South Atlantic as we pass from lower to higher 

 latitudes ; so that to the southward of the parallel of 42° S. the summer 

 and the winter climates differ comparatively little from one another, whilst 

 near the south point of South America the climate is remarkable for its 

 mildness in winter. 



118. Now these general differences between the Thermal conditions of 

 the Northern and the Southern Atlantic are fairly attributable to that 

 great preponderance in the proportion of Land to Sea in the Northern 

 Hemisphere, which is fully recognized by all Physical Geograpners and 

 Meteorologists as an adequate vera causa alike for the higher average 

 Temperature of the Northern Hemisphere as a whole and for its greater 

 annual range*. This preponderance is particularly marked in the Eastern 

 Hemisphere, where, in the 180 degrees of Longitude which include the 

 whole of Europe and very nearly the whole of Asia, the Thermic 

 Equator is almost a great circle inclined at an angle of 15° to the Geogra- 



for those of Latitudes beyond 50° Jsorth, I rely on the two Charts accompanying Dr. 

 Petermann's Memoir. 



* See Sir John Herschel's 1 Physical Geography,' §§ 250-264. 



