48 PRACTICAL LESSONS IN SCIENCE. 



is greatly retarded by friction in the pipes, so that much larger 

 pipes should be used than might at first seem necessary. 



Motions in the air and other gases are very common, 

 in fact the air is seldom or never at rest. The differences in 

 temperature between day and night, between the sea and 

 land, between forest and plain, all tend to cause inequalities 

 of pressure which result in more or less extensive motions, 

 which vary from gentle, almost imperceptible, breezes of two or 

 three miles per hour, to the fierce and destructive tornado in 

 which the velocity of the air is as great as 50, 75, or even 100 

 miles per hour. The difference of temperature between the equa- 

 torial and polar regions is perhaps the main cause of the general 

 system of winds of the globe, the rotation of the earth influ- 

 encing the direction of the air movement. If a fire is built, the 

 air near it is heated, and expanding becomes less dense than the 

 surrounding air. The heavier air crowding in from all directions 

 forces the lighter air upward, giving rise to currents of air from 

 all directions toward the fire, all of which unite in an upward 

 current from the fire. The great heat of the equatorial regions 

 causes the air of those regions to be less dense than that of regions 

 north or south. This gives rise to a movement of air from both 

 directions toward the equatorial regions, forming there an up- 

 ward current, which, spreading out, flows back toward the polar 

 regions as counter-currents, so that the air of the earth is in con- 

 stant circulation in a general way, besides the thousands of minor 

 movements due to local causes. If the earth did not rotate we 

 might expect these currents to move directly north and south ; 

 but the spherical form and rotary motion of the earth change the 

 directions somewhat. On account of the form and rotation of 

 the earth, those places near the equator move more rapidly than 

 those either north or south of the equator. Places on the equa- 

 tor have a motion of 1,042 miles per hour, while those at 20 N. 

 or S. have a motion of only 975 miles per hour. Air, moving 

 toward the equator, having the slower motion of places north 

 and south, falls behind places at the equator having the same 

 longitude as those from which it started, so that these currents 



