146 THE MICHIGAN ACADEMY OF SCIENCE. 



from comparing the increased weight with the average weight of the 

 atmosphere at that place, because the increase is due to the weight which 

 has been moved from that part of the earth over which the sun has been 

 shining, — that part which is caused to rise as the heavih-weighted part 

 falls eastward toward the sun. 



Here are diagrams representing the atmospheric pressure at each hour 

 of the da}' and night, from which 3'ou may see the fluctuations from hour 

 to hour. 



At first thought, one might wonder whether this comparatively small 

 (nine pounds per s(|uare foot of surface) extra Aveight of the atmos- 

 phere massed especially in equatorial latitudes and along lines of longi- 

 tude approaching the sun, and the co-incident lessened weight along lines 

 of longitude receding from the sun may not be greatly overbalanced by the 

 weight of some mountain range in some other loitgitude. But this idea 

 is not applicable, because of the manner in which this extra atmos- 

 pheric Aveight is applied to the earth, it being applied to mountain sides 

 and all parts of the earth in rapid succession, very much as is our cus- 

 tom of rotating a geographical globe by pressing from time to time on its 

 equatorial part in the direction we wish it to turn. The analogy to the 

 force thus applied by the hand is very close, because the atmospheric 

 pressure increases gradually from sunrise until about ten A. M.. and 

 then decreases very rapidly, being less than the average by two P. M., 

 b}' which hour the wind is usually at its nmximum and its prevailing 

 direction in the temperate zones is eastward, toward that part of the 

 earth where it is then afternoon. 



This diagram exhibiting the average A'elocity of the wind at Lansing, 

 Michigan, at each hour of the day, illustrates to some extent that ''the 

 wind rises and goes down with the sun," but the greatest velocity of 

 the wind is not at the hour of greatest atmospheric pressure (ten A. 

 M.) but later, — from twelve noon to two P. M. 



Although at any given place the effect of the prevailing wind toward 

 the rotation of the earth appears mainly during the day time it is 

 really eontinuous,^it is day time at some part of the world coustantl3^ 

 At any given place, its greatest effect is just after the extra weight of 

 the atmosphere has ceased to have its effect in the same direction. 



It is apparent, I think, that the movement of the atmosphere which 

 is caused by the sun and which we recognize as wind, exerts a consider- 

 able force tending toward the eastward rotation of the earth. Also that 

 the movement of the atmosphere which is caused by the sun and is not 

 apparent at the surface of the earth, except as pressure measured by 

 the barometer, exerts a force tending also toward the eastward rotation 

 of the earth. 



My suggestion is that, together, the Avind and the daily variation of 

 the atmospheric pressure, these two results of the radiant energy re- 

 ceiA^ed from the sun constitute the force which maintains the rotation of 

 this earth, notwithstanding the effect of the tidal and possibly other 

 influence of the sun and moon to retard that rotation. 



That the moon has no other rotation than that connected Avith its 

 revolution around this earth, I believe may be due to the fact that it 

 has no atmosphere to be thrown out of equilibrium by the heat from the 

 sun. If this earth had no atmosphere, possibly it might constantly pre- 

 sent to the sun the same side of itself in like manner as the moon pre- 

 sents the same side of itself to the earth. 



