Mr. W. Brown on the Oscillations of the Barometer. 285 



Now the south wind descends or blows in the greatest de- 

 gree during the winter half of the year *, and then most at 

 night; that is, when the temperature of the earth and the at- 

 mosphere is falling by radiation; whilst in spring and summer 

 — the temperature increasing — the lower current, or north- 

 east wind, prevails more than at any other time of the year. 

 Now it is obvious that were the cooling of the atmosphere un- 

 affected by that of the earth, its upper strata, notwithstanding 

 the facility with which heat passes through it, would be first 

 and most cooled by radiation ; but we know that within small 

 heights — to the extent of about 100 feet — above the surface of 

 the earth, in which such observations can be easily conducted, 

 the lower strata are much the coldest in nights when radiation 

 is vigorous, by reason of the cooling of the earth. It is ob- 

 vious however that it does not at all necessarily follow from 

 this that this increase of temperature in ascending, or rather 

 as it would be after a certain height, increase compared with 

 the general progressive decrease of the temperature, should 

 go on towards the higher regions of the atmosphere; for it is 

 evident, from the very great rapidity in which the temperature 

 decreases towards the surface, that it is very much, if not al- 

 most entirely, owing to contact ; but on the contrary there 

 will be a certain elevation, perhaps not very great, at which 

 the cooling of the atmosphere by the greater radiation of heat 

 into space in the strata above than in those below it, is so 

 much greater than the cooling in the air below, occasioned 

 by radiation to the earth, that the previous relation of the 

 temperatures of the air of the two currents may become at 

 any time, when this difference is not very great, reversed, and 

 the upper one, being then comparatively the colder, descends. 

 The cooling may also be materially affected by the presence 

 of clouds, and by the heights above the surface of the earth 

 at which they are formed; as these would present a compa- 

 ratively dense radiating body, and by cooling the particles of 

 air in contact with them, would cause currents of cold air to 



* The blowing of the wind from west, whether north or south, may of 

 course (except in the case (§ 12) where it is caused by the flowing of air 

 to restore the atmospheric pressure as in storms) be taken as evidence of 

 the descent of the upper current; thus in the cold months of the year, 

 although the north wind is strongly urged on towards warmer regions by the 

 greater difference of temperature between adjacent latitudes, due to the 

 season, and frequently prevails, — it is generally from north-west. 



The phenomena of tropical regions correspond to those of high lati- 

 tudes ; thus the time of descent, or of hurricanes, is after the sun has at- 

 tained its greatest northern declination, consequently when solar radiation 

 is decreasing ; the temperature however is very little fallen, and is in some- 

 what of the variable state of high latitudes, the blowing of the trade-wind 

 being then nearly suspended. 



