PROBLEMS IN TERRESTRIAL PHYSICS. 147 



THE DIURXAL OSCILLATIONS OP ATMOSPHERIC PRESSURE. 



The reason why the earth's atmosphere banks up as it does in the 

 forenoon, notAvithstauding- the fact that it is perfectly free to flow off 

 in nearly every direction, is because of its inertia and the great suddenness 

 with which the heat of the sun is applied to it, the rate of movement 

 being about one thousand miles an hour ; slightly more than that at the 

 equator, and slightly less in this latitude. The outflow from the area 

 of the sun's raj'S begins at once — ^the wind rises with the sun; but that 

 it does not keep pace with the heating and banking up of the atmosphere 

 is proved by the fact that the maximum wind is later than the maximum 

 pressure b}' rather more than two iiours. This represents one result of 

 the inertia of the atmosphere. Another secondary banking up of the 

 atmosphere, I believe, results from its inertia when, falling as wind from 

 the forenoon "high"' to the afternoon ''low,'' it continues up the gradient 

 beyond the low, and manifests itself as the 10 P. M. high pressure. At 

 least I believe that the 10 P. M. maximum atmospheric pressure is a 

 result, in great part, of the afternoon wind — it is a wave of atmosphere 

 which rises up the incline to above the cold, dense night air, by reason 

 of its momentum, as a pendulum rises after having fallen from the 

 opposite direction. Accordingly it is a similar but smaller wave than 

 the 10 A. M. high pressure, because it has not like that the accumulating 

 force of the sun's heat to cause it to accumulate, and it has lost some- 

 what by friction with the earth's surface. The wave is a comparatively 

 stable one, — remaining somewhat constantly in the same position rela- 

 tive to the sun. Probably the extent and time of the 10 P. M. maximum 

 will be found to vary with the force of the afternoon wind — a strong 

 wind and long daylight increasing or postponing the time of the maxi- 

 mum night pressure. 



THE EARLY MORNING MINIMU:M PRESSURE. 



Just how the early morning minimum atmospheric pressure is caused 

 has been to meteorologists a puzzling question, ''There is no doubt that 

 the morning minimum is the most elusive of all the barometric phases." — 

 J. R. Sutton, A. A.. F. R. Met. Soc, in Quarterly Journal of the Royal 

 Meteorological Society, January, 1901. In this latitude, from about mid- 

 night to about 3 or 4 A. M., the pressure becomes gradually less, not- 

 withstanding the fact that there is, to account for the change, no great 

 movement of the atmosphere near the earth's surface such as should be 

 associated with high pressure both west and east of the morning mini- 

 mum, namely about 10 P. M. and 10 A. M., the low pressure being about 

 4 A. M.- It follows that the movement is probably in the extreme upper 

 atmosphere. Its explanation seems to me to be that in the early morn- 

 ing hours the upper air moves toward the sun, because of the sun's 

 attraction. This may be appreciated by considering first the accepted 

 causation of twilight, — by the illumination of the upper regions, while 

 the sun is below the horizon, the light being irregularly reflected by the 

 molecules of the atmosphere. So long as the upper regions of the atmo- 

 sphere are thus illuminated so long is the atmosphere in those regions 

 being acted on, not by the radiant energy which readily passes through 

 it, but by the attraction of the sun; and, being perfectly free to move, 

 the friction being exceedingly slight, the earth's attraction offering little 



