PROBLEMS IN TERRESTRIAL PHYSICS. 137 



weight toward areas of less weight of the atmosphere. This is a general 

 law which has important bearings upon many questions in meteorology, 

 and I believe also in terrestrial physics. 



The important cause of areas of high and areas of low barometer is 

 the heat received from the sun, the amount of which dififers greatly in 

 different localities. The first effect of heat applied to the air itself is 

 to expand the air and to increase the pressure in every direction.* When- 

 ever the direction of least resistance is upward, the air rises, and when 

 extending higher than the air over surrounding areas the rarified air 

 tends to flow off over other areas thus causing high, or at least in- 

 creased, pressure in those other areas, the rapidity of such overflow 

 depending upon the gradient. As soon as the expansive force of heat 

 has passed its maximum and an equilibrium of height of the atmos- 

 phere is established, as occurs ordinarily soon after noon each day, the 

 area which has been heated by the sun will be an area of low atmos- 

 pheric pressure. The force of the wind will be appreciated not so much 

 in the area of high pressure as between that area and the area of low 

 pressure. Therefore, although "the wind rises with the sun" it reaches 

 its maximum after the sun has passed its point of maximum heating 

 effect. 



In Michigan the average daily range of temperature, — the difference 

 between the mid-day and the night temperatures, in the twenty-two 

 years ending with iooo, was eighteen degrees Fah. The expansion of 

 air by heat, is about two one-thousandths of its bulk for each degree 

 Fah. Therefore the average expansion of the air near the earth's sur- 

 face in Michigan each day on the average was and is (.0020361X18 

 =.0365898) a little less than four one- hundredths of its bulk. If we 

 assume that our atmosphere extends upwards, on the average, one hun- 

 dred miles, and also assume hypothetically that the expansion caused 

 by the sun extends all that distance, that nearly all of the expansion is 

 in that direction, and that the expansion continues to be as great 

 throughout, then each day near mid-day there is added to the average 

 height of our atmosphere nearly four miles. The problem is not as 

 simple as these assumptions might imply, because the pressure and tem- 

 perature are so very much less in the upper regions, and because the 

 movement is not by any means nearly all upward; but it is, I think, 

 apparent that there are good reasons for believing that the air flows off 

 from the sun-heated areas, and partly because its height is considerably 

 increased by expansion. 



It being impossible for the sun's rays to penetrate the earth as they 

 do the ocean to very great depths, over land areas the heat is almost 

 wholly used in expanding and rarifying the atmosphere, and the reason 

 why the extent of the effect differs locally even over the land is because 

 of the difference in the earth's surface locally, in some places having no 

 covering, in others being covered with high and dense vegetation, in the 

 one case there being intense heat, in the other case great cooling influ- 

 ence through the evaporation of water from high trees whose roots ex- 

 tend deep into the earth and bring therefrom great quantities of water 

 to be evaporated from millions of leaves. The influence, locally, of high 

 hills and mountains preventing the free movement of the atmosphere, 



*The coefficient of expansion of air is 0.00367 for one degree C, or 0.0020361 for one degree Fah. 

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