l 286 Mr. W. Brown on the Oscillations of the Barometer. 



descend; and also prevent radiation from the earth and atmo- 

 spheric strata beneath them. Admitting such a cause of de- 

 scent of cold air, we shall have an explanation of those sudden 

 colds often experienced, which cannot be accounted for on the 

 supposition of the arrival of air from colder regions. 



But in the summer half of the year the time of descent of 

 the upper current is not in the night more especially than 

 during the day. 



Being desirous of confirming the opinion that the south 

 wind prevailed most at night, I selected from my own register 

 of the wind, made three times a day, all the changes of wind 

 from north to south and from south to north for one year, 

 noting the time (whether night or day) at which they oc- 

 curred, and met with the unexpected result, that whilst with 

 regard to the winter months the opinion was amply confirmed, 

 in those of summer the result was rather the opposite to it, 

 the change on the whole taking place most frequently in the 

 daytime ; and in order to confirm this conclusion, the changes 

 were selected from two other years, with the same result. I 

 have presented in a table the average number of changes 

 for the three years, merely remarking that with scarcely an 

 exception the same month of each of the three years gave a 

 result in accordance with the mean one. The table also con- 

 tains the changes from south to north, which are in general 

 opposite to the contrary ones. 



The want of exact consistency which appears in the results 

 of the summer months is easily accounted for by the lightness 

 of the wind and its arising often from merely local circum- 

 stances ; the total result however is sufficiently decided to 

 show the difference of character of the winter months. Now 

 in summer the temperature of the lower strata of the atmo- 

 sphere with respect to those above, from the heat derived 

 from the ground, is comparatively much greater than in winter, 



