REPORT ON ATMOSPHERIC CIRCULATION. 23 



The most noticeable feature of these daily barometric oscillations is their very large 

 amounts, those at Gries, though in lat. 46° 30' N., being tropical in amount; and the 

 singular circumstance is that in no season does the morning minimum fall so low as 

 the daily mean. Gries, Klagenfurt, and Cordova are each situated in a deep valley, 

 and they present the diurnal barometric curves characteristic of these places (Plate I. 

 fig. 20). In such situations, during night, the whole surface of the region is cooled 

 by radiation below the air above it, and the air in immediate contact with the ground 

 becoming also cooled, a system of descending air-currents sets in over the whole face of 

 the country bounding the deep valley. The direction and velocity of these descending 

 currents are modified by the irregularities of the ground, and, like currents of water, 

 they converge in the bottom of the valleys, which they fill to a considerable height 

 with the cold air they bring down from the sides of the mountains. This cold and 

 consequently relatively dense air rises above the barometers which happen to be down 

 in the valley, with the result that a high mean pressure is maintained during the 

 night. In summer the pressure at the coldest time of the night is maintained, 0"020 

 inch, at Klagenfurt, higher than it is in open situations in that country, and double this 

 amount, or 0"040 inch, at Gries. On the other hand, during the day these deep valleys 

 become highly heated by the sun, and a strong ascending current is early formed, under 

 which pressure falls unusually low. Thus, while at Vienna the afternoon minimum 

 falls 0'026 inch below the daily mean, at Klagenfurt the amount is 0'042 inch, and at 

 Gries 0-058 inch. 



The same feature of the pressure is seen, though in a much less pronounced degree, 

 in the curves for Mexico, where the daily range is usually large for so elevated a station 

 and consequent low mean pressure, and where the morning minimum either does not 

 fall to the mean of the day or but little below it. 



On the other hand, at high-level observatories, such as Obirgipfel and Ben Nevis, 

 which are situated on true peaks, the daily curve of pressure is wholly different 

 (Plate I. fig. 21). In these situations the curves all show an abnormally large 

 morning minimum, and, in summer more particularly, an afternoon minimum so small 

 as all but wholly to disappear. 



It follows that the diurnal curves of atmospheric pressure are liable to large modifi- 

 cations according as the earth's surface, in the more immediate neighbourhood of the 

 barometer from which the observations are made, presents a level plain, a troughed 

 hollow between mountains or rising grounds, or an isolated peak. 



In high latitudes, in the interior of continents, when there is either constant 

 sunshine, or sunshine and a strongly pronounced twilight, the morning minimum is 

 much reduced, and in the height of summer vanishes altogether, being probably the 

 effect of the short nights, the comparatively slow motion from the earth's rotation, and 

 the constant heating from the sun's rays, direct or indirect. The summer curve for 



