CLIMATE. 45 



another in mutual interplay. Between these is, in mid-ocean, a 

 region of calms. The extratropical winds also in part pass on to 

 the poles, to return, as northeast, north, and northwest winds, 

 towards the equator. 



The cause of the motion is not now considered, as it is here in place only 

 to present in a comprehensive manner the earth's exterior features. The 

 causes varying the directions consist in — (1) the temperature of the land and 

 ocean ; (2) the form of the land (mountains being barriers to a flow, retarding 

 by friction, etc.) ; (3) difference of density of cold and warm air; (4) changing 

 seasons, etc. But these sources of disturbance only modify without suspend- 

 ing the system of movement. 



47. Climate. — Climate is determined by the atmospheric and 

 oceanic movements and the distribution of land and water. The 

 existing system may be briefly explained, in order to complete this 

 survey of the earth's physiography. 



1. The land takes up heat rapidly in summer, and, in the north, 

 becomes frozen and snow-clad in winter. Land-winds may, conse- 

 quently, be intensely hot or intensely cold ; and hence lands have 

 a tendency to produce extremes of climate. 



A place on the continents having a mean January temperature of 50° (a very 

 warm temperature for that season) is to be found only in warm latitudes, and 

 one with a mean July temperature of 50° (a cold temperature for the season) 

 only in the colder zones of the globe. The mean January temperature of New 

 York is 3I5 F., while the mean July temperature is 73°. Now, in North Ame- 

 rica the January isothermal line of 50° almost touches the Gulf of Mexico, and 

 the July line of 50° passes near the mouth of Mackenzie River, or the arctic 

 circle, — the extreme winters and intense summers causing this great change. 

 In Asia, again, the January line of 50° runs just north of Canton, near 26° N., 

 and the July line of 50° touches the Arctic Ocean at the mouth of the Lena, in 

 72° N., making a difference of 46° of latitude, or nearly 3000 miles, as the effect 

 of the land on the climate. 



2. The waters of the oceans remain unfrozen even far into the 

 Arctic, unless crowded with lands, their perpetual movements tend- 

 ing to produce a uniformity of temperature over the globe ; and 

 hence winds from the oceans or any large body of water are 

 moderating, and never very cold. They produce what is called an 

 insular climate. 



Great Britain is tempered in its climate by its winds and the oceanic current 

 (the Gulf Stream). Fuegia, which is almost surrounded by water, also has an 

 insular climate, — the winter's cold falling little below 32°, although below 56° 

 S. latitude. 



3. Absence of land from high latitudes is equivalent to an 

 absence of the source of extreme cold ; and from tropical lati- 



