144 THE MICHIGAN ACADEMY OF SCIENCE. 



at the earth's surface and high above it is, say, forty miles per hour, then 

 that is the force exerted upon the earth, and the pressure would be about 

 eight pounds per square foot of surface exposed to the wind. But not 

 all of the pressure would tend to rotate the earth eastward, although 

 the prevailing tendency is in that direction. The proportion of that ten- 

 dency in temperate regions may be estimated from the preceding table. 

 Although the force is applied to the extreme surface, where it has the 

 greatest effect, it is worthy of notice that it is not applied to all parts 

 of the earth all the time, because ''the wind goes down with the sun," 

 and it usually remains absent or slight until about sunrise when it 

 increases until its maximum, about two o'clock in the afternoon. This 

 condition of sunshine and consequent wind, over nearly an entire hem- 

 isphere, goes round the earth each day, or, niore correctly speaking, 

 the earth by its rotation, daily subjects the greater portion of itself 

 around its entire circumference to the sunshine and consequent wind, 

 the prevailing direction of which in the two gTeat temperate belts is 

 eastward. 



It is true that the eastward movement of the atmosphere is mainly in 

 the temperate zones, the prevailing surface winds at the equator not 

 being in that direction. My view of the effect of this may be illustrated 

 by a grindstone turned by a crank, the handle of which does not extend 

 outward as far as the circumference of the stone, yet which receives 

 sufficient force and rotates the stone notwithstanding the friction of 

 the tool being sharpened on its circumference. And the surface winds 

 either at the equator or in the tropics do not supply much resistance to 

 the earth's rotation. At the equator, over four or six degrees of lati- 

 tude, is the region of calms, where the main surface movement of the 

 air is upward as the sun is brought to heat and expand it ; and on either 

 side of this region the westward movement of the air is steady but not 

 forceful. Prof. Arnold Guyot has stated that these trade winds move 

 at the rate of fifteen to eighteen miles per day.* 



In the polar regions the eft'ect of westward movement of atmosphere 

 toward retarding the rotation is much less than an equal force would 

 be if it were applied nearer the equator. 



Even if we assume that the force exerted on the earth by the winds 

 in the tropical and in the polar regions, tending to retard the earth's 

 rotation is as great per square foot of area as it is in the temperate 

 regions tending to accelerate the rotation, there is still apparent a 

 large surplus of the accelerating force, because of the greater propor- 

 tion of land, of uneven surface, and less proportion of water, of level 

 surface, in the temperate regions, and also because of the greater total 

 area in the temperate regions. Thus, assuming the force exerted on the 

 earth by the wind to be eight pounds per square foot, and that one-half 

 of that force tends in the tropical and polar regions to retard, and in 

 the temperate region to accelerate rotation, then, as the area in the 

 temperate regions exceeds that in the tropical and polar regions com- 

 bined,! the total force exerted in the temperate regions would exceed 

 that in the combined other regions bv over 420,000,000,000 of tons. 

 (Square miles in temperate regions=l 02,221,526 — 94,678,752, square 

 miles in tropical and polar regions=7,542,774,:^excess in temperate 

 region X 55,756.8,— tons per square mile=420,560,941,363). 



♦Johnson's Encyclopedia, Vol. IV, p. 1-146. 

 t Johnson's Encyclopedia, Vol. I, p. 979. 



