B. TERRESTRIAL PHYSICS AND METEOROLOGY. 107 



in December, 1873, he addressed letters to many observers 

 in Europe, soliciting accurate observations on the move- 

 ments of the cirrus clouds. All the observations were care- 

 fully entered by him in the weather-charts of Europe, and 

 compared with the observed direction of the lower clouds, 

 as well as the course of the isobarometric lines. From the 

 entire series of observations accumulated in three years, Mr. 

 Hildebrandsson gives his conclusions as follows : Near the 

 centre of a barometric depression the upper currents move 

 in a direction very nearly parallel to the isobars and to the 

 lower currents ; but in proportion as we go from the centre 

 of low pressure they move outward, and deviate to the right 

 of the lower clouds. Within regions of maximum barometric 

 pressure the upper clouds converge toward the centre, cutting 

 the isobars very nearly at right angles. The figures prove 

 clearly that the upper currents of the atmosphere move from 

 the barometric minima, and converge toward the maxima. 



In respect to the arrangement of parallel bands of cirrus 

 clouds, our author states that he has inserted upon his charts 

 171 observations, and finds that the bands of cirri are, within the 

 regions of maximum pressure, generally arranged in directions 

 very nearly perpendicular to the isobars, but in the regions of 

 minimum pressure they are sensibly parallel to the isobars. 

 It follows from his investigation that the air must necessari- 

 ly be ascending in the region of low barometer, and that, when 

 arrived at a great height above the earth, it removes from this 

 region of low pressure and, spreading uniformly out over the 

 regions of maximum pressure, gradually falls toward the 

 earth in descending: currents. In this manner there is ef- 

 fected a continual vertical circulation between the surface 

 of the earth and the upper limits of the atmosphere. The 

 principal agent in this circulation is the difference in tem- 

 perature and moisture between the lower and the up- 

 per regions of the atmosphere differences which exceed 

 even that between the equator and the poles. Carrying 

 out this idea, Hildebrandsson concludes that the cold ex- 

 perienced at the surface of the earth during areas of high 

 pressure comes from above, being brought down by the de- 

 scending atmosphere, and explains thereby the anomalous 

 daily period in the determinations of the altitudes by the 

 barometer. The conclusion of his memoir, expressing the 



