XX.] THE MOVEMENTS OF THE EARTH. 343 



something like 1,000 miles an hour. But, on going either 

 north or south from the equator, the circle which is described 

 by any point on the rotating sphere will be smaller, as is 

 shown by the diminution in the diameter of the circles of 

 latitude. Yet every point of the surface takes the same 

 time to turn once round ; and, therefore, the velocity, or 

 rate of motion, must become less and less, as the circles 

 get smaller and smaller. In fact, at the poles, the velocity 

 is reduced to nothing. The pole represents simply the end 

 of the imaginary line on which the earth turns, and is itself 

 stationary. 



Everything on the surface of the earth is necessarily 

 carried round with the rotating globe. The atmosphere, as 

 shown in Chapter VI. may be regarded as- part and parcel 

 of the earth; it forms, in fact, a gaseous shell, which com- 

 pletely encases the globe, and shares in all its movements. 

 The atmosphere, therefore, moves round at the same rate 

 as the surface on which it rests. But this surface rotates, 

 as just explained, at different speeds, in different latitudes ; 

 and hence the atmosphere, while quiescent over the poles, 

 moves with increasing rapidity in lower latitudes, until it 

 attains 1000 miles an hour at the equator. Therefore, if a 

 stream of air starts from one of the poles towards the 

 equator, and moves in a direct north-and-south line ; that 

 is to say, along a meridian, it will constantly tend to lag 

 behind the surface of the earth. At the starting point, the 

 air is stationary, because the pole itself has no motion ; 

 and, if we could suppose such a stream of air to flow due 

 south without coming into contact with anything, the suc- 

 cessive points of the earth's surface over which it passed, 

 would turn under it with constantly-increasing swiftness ; 

 until, at the equator, they would whirl by at the rate of 

 1, 000 miles an hour to the east. Imagine the air thus 



