DISCUSSION AND ANALYSIS OF WINDS. 667 



reference to the maps shows that, in the mam, snch is the actual arrangement of 

 pressures on the oceans and on parts of the continents. 



It is easy to see that this is the general conclusion arrived at by Prof. Coffin in 

 his " Winds of the Northern Hemisphere." The main result is thus the same, the 

 study of the winds, alone having shown that this is the case in a great part of the 

 globe, Avhile what we have said as to the pressure of the air shows at least the 

 proximate cause of the prevailing winds. In hoAV far this normal arrangement of 

 winds is disturbed by geographical features, especially by the influence of the 

 continents, will be shown later. 



A further condition is the yearly movements of the belts of high and low pres- 

 sure with the change of seasons. When the sun is in the zenith over the northern 

 hemisphere, the seas under it will be more heated than the southern seas, and the 

 equatorial belt of low pressure, which is also on the seas, the belt of highest 

 temperature, will move northward. Owing to the great specific heat of the water, 

 and consequently to the longer time it takes to cool, this northward movement will 

 continue nearly to the end of the summer. On the other hand, the belt of low 

 pressure in the higher latitudes will also move northward as the temperature rises 

 near the poles, and the storm-tracks can take a more northerly course. The belt 

 of highest pressure between the two of lowest must also take a more northerly 

 position, as the air floAvs both north and south out from it. There can be no doubt 

 that it holds an intermediate position between the two. 



When the sun is in the zenith over the southern hemisphere, the reverse takes 

 place : the equatorial belt of lowest pressure recedes southward, and also that in 

 higher latitudes of the northern hemisphere, as the polar regions are so much cooled 

 that the condensation of vapor there cannot sustain great barometric depressions. 

 These normal or ideal conditions are realized to some extent on the surface of the 

 present oceans, and are the more striking, the larger the bodies of water are. Gene- 

 rally the southern hemisphere has meteorological conditions which approach more 

 nearly to the normal conditions than the northern. Thus, it will be seen by refer- 

 ence to the map of the isobars that the high pressure in about 30° really encircles 

 the globe in the southern hemisphere, while in the northern, the pressure is highest 

 ia January at about latitude from 50° to 53° N. in Asia, and in July the pressure 

 is very low, about 30° L. N. on the same continent. Again the low pressure about 

 from 60° to 65° encircles the globe in the southern hemisphere, the difiierence of pres- 

 sure under the difi"erent meridians not being great, and further south (especially 

 from 70° to 78°) somewhat higher pressure and easterly winds are found. In the 

 northern hemisphere, on the contrary, the lowest pressure is found on two elliptical 

 spaces, in the Northern Atlantic, about Iceland, and in the Northern Pacific, about 

 the Aleutian Islands, that is, where a great extent of water prevails at about 60°, 

 and the ocean is abnormally heated by currents of warm water. 



We thus see that at a distance from the influence of water, the above-stated 

 normal conditions are very much interfered with. 



If the earth consisted mainly of continents without intervening oceans, very 

 diff'erent conditions would prevail. As continents are more rapidly heated than 

 oceans, temperature would be highest very soon after the passage of the sun 



