138 THE MICHIGAN ACADEMY OF SCIENCE. 



also accounts for some of the unequal effects of the sun in causing at- 

 mospheric movements. 



One not familiar with the subject might imagine that areas of high 

 and low barometer are scattered about with no system of regularity; 

 but while this is true to a considerable extent, as a rule there is great 

 regularity in the movement of the atmosphere from west to east, in the 

 temperate zones. 



The following table copied from Silliman's Principles of Physics, ex- 

 hibits the relative frequency in the principal countries of the northern 

 hemisphere, of winds from different points of the compass. The total 

 number of winds in each country is represented by 1,000 : 



FREQUENCY OF DIFFERENT WINDS. 



"In all the northern hemisphere there is a predominance of westerly 

 winds. This is shown by the fact that the average length of the voy- 

 age from New York to Liverpool by packet is but twenty-three days, 

 while that of the return voyage is forty. In high southern latitudes 

 the same thing is observed. Lieut. Maury remarks that at Cape Horn 

 there are three times as many westerly as easterly winds,"* That is, 

 from some westerly point. 



It has been thought that the general movement of the earth's atmos- 

 phere from west to east is due to the rotation of the earth from west to 

 east; but if so, why does the atmosphere move faster than the earth? 

 The earth rotates its twenty-five thousand miles of surface, at the 

 equator, in twenty-four hours, — a little over one thousand miles per 

 hour, and the wind averages several miles per hour faster than that, 

 because the prevailing direction of the movement is much the same as 

 the direction of rotation, and considering the earth as fixed and sta- 

 tionary the upper atmosphere at the equator moves eastward faster 

 than the earth. 



The mid-day and afternoon wind, and its causation by the forenoon 

 atmospheric pressure, may be better appreciated by a glance at the two 

 diagrams illustrating this subject. 



The wind blows from areas of high pressure toward areas of low pres- 

 ure; therefore, ordinarily, the wind blows from areas where it is fore- 

 noon toward areas where it is afternoon. As the condition of forenoon 

 is constantly advancing from the east toward the west, there is an area 

 of forenoon high pressure constantly advancing westward, followed by 

 an area of afternoon low pressure. Consequently, at some portion of 

 that very broad belt around the world on which the sun shines, there 

 is constantly, as a rule, a wind blowing toward that area of afternoon 

 low pressure; and in the temperate regions, not only the upper but also 



♦Principles of Physics, Silliman, p. 646. 



