PROBLEMS IN TERRESTRIAL PHYSICS. 145 



But the force exerted by the wind on the inequalities at the earth's 

 surface is not the only force exerted by the atmosphere tending toward 

 eastward rotation. This, however, can be better understood after noticing 

 again why the prevailing wind is eastward, and why there is so gen- 

 erally each day about noon toward the west an area of high pressure 

 and toward the east an area of low pressure. (Diagram). 



Holding in mind the fact that the heat from the sun expands the air 

 at the earth's surface, some of which air, rising flows off over regions not 

 so heated and where the lower atmosphere is more condensed, thus caus- 

 ing high pressure, we can readily see that the sun causes continuously 

 high pressure on a portion of the earth's surface as fast as any portion 

 of it comes under its heating influence, — namely, from sunrise until 

 noon. 



This brings us to consider the force exerted on this earth by its atmos- 

 phere by reason of its weight, in connection with its movements not 

 appreciated by us as wind because the movement is high above the reach 

 of our ordinary observation. To appreciate this movement it is neces- 

 sary to consider the weight of the atmosphere from hour to hour during 

 the da}' and during the night. 



The extent to which the atmosphere flows off from a heated area over 

 the condensed atmosphere of colder regions, can be measured by means 

 of the barometer, hourly observations of which teach us that there is a 

 great wave of the atmosphere which goes surging upward, to high above 

 the earth, advancing westward just as fast as the earth rotates from 

 west to east, namely, about a thousand miles per hour, so that the high- 

 est crest of the wave is always opposite any given place in this latitude 

 at about ten o'clock in the morning. ^ 



The barometer informs us of the weight of this extra load of atmos- 

 phere on the descending quadrant of the world, beginning about sunrise 

 and ending about noon. Near the equator, over continents of land, it 

 amounts to about nine pounds per square foot more than the minimum 

 pressure.' We know this because the average pressure for all hours of 

 the day at the sea level sustains a column of mercury weighing nearly 

 fifteen pounds per square inch, or about one ton per square foot.* 



The increased pressure in the forenoon is great near the equator, and 

 it decreases toward the poles. Hourly observations at Silchar, in 

 Assam, near the equator, showed that the difference between the high 

 pressure in the forenoon and the low pressure in the afternoon is .133 

 of an inch; at Singapore, on the equator, but surrounded by water, it is 

 .12 of an inch ; at Bombay, in January, the variation was nearly .08 of an 

 inch of mercury; at Calcutta (latitude 22° — 35' N.) it av'eraged about 

 five one-hundredths ; at Toronto, Ontario (still further north), it aver- 

 aged not quite three one-hundredths; and at Polaris Bay (latitude 

 81° — 36" N.) in December, the rise of pressure at those hours of the day 

 was almost imperceptible, although there was a slight rise in the early 

 morning hours, — about three A. M. 



The amount of the extra weight of the atmosphere shades off not only 

 from the equator toward the poles, but also each way from about ten 

 o'clock in the morning, its beginning being about sunrise, its maximum 

 about ten A. M., and passing below the average about one P. M. 



The effect toward rotation of the earth is more than would appear 



*(30 in. - 15 lbs.; 1 in. = ^4 lb. per sq. in.; H4 sg. in. -^ y^ - n lbs.; 72 lbs. X .13 = 9.36 lbs.) 

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