1844.J/W a paper read before the British Association 1845.1 11 



The very slight irregularity which shews itself at the hour of noon re- 

 quires to be confirmed by the observations of subsequent years before 

 can be regarded as possessing a systematic character. When, however, 

 we direct our attention to the gaseous pressure, we perceive very distinct- 

 ly marked the characters of a double progression, having one maximum 

 at 10 h , and another at 22 h ; one minimum at 4 h , and another at 16\ The 

 double progression is exhibited both in the clouded and in the clear seasons 

 with a slight difference only in the hours of maxima, the principal maxi- 

 mum in the cloudy season being at 20 h instead of 22 h , and in the inferior 

 maximum in the clear season being at 12 h instead of 10 h . The range of 

 the diurnal variation, like that of the temperature, is more than twice as 

 great in the clear as in the clouded season. 



If we now turn our attention to the phenomena of the direction and 

 force of the wind, we find by Dr. Buist's report that, for two hundred 

 days in the year, there is a regular alternation of land and sea-breezes. 

 The land-breeze springs up usually about 10 h , or between 10 h and 14 h , 

 blows stronger and fresher towards daybreak, and gradually declines un- 

 til about 22 h , at which time the direction of the aerial currents change 

 and there is generally a lull of an hour or an hour and a half's duration. 

 The sea-breeze then sets in, the ripple on the surface of the water, indi- 

 cating its commencement, being first observed close in shore, and extend- 

 ing itself gradually out to sea. The sea-breeze is freshest about 2 h , and 

 progressively declines in the evening hours. The diurnal variation in the 

 force of the wind during these 200 days is therefore obviously a double 

 progression, having two maxima and two minima — one maximum at or 

 near the hottest, and the other at or near the coldest hour of the day ? 

 being the hours when the difference of temperature is greatest between 

 columns of air which rest respectively on the surface of land and sea, and 

 two minima coinciding with the hours when the surface temperature over 

 the land and over the sea approaches nearly to an equality. 



In the remaining portion of the year, the diurnal range of the temper- 

 ature is most frequently insufficient to produce that alternation in the di- 

 rection of the wind which prevails uninterruptedly during the large por- 

 tion. There appears, however, to have been only one month, viz. July, 

 in the year 1843, in which there were not some days in which the alter- 

 nation of land and sea-breezes was perceptible. The causes which pro- 

 duce the alternation are not therefore wholly inoperative, though the ef- 

 fects are comparatively feeble during the clouded weather which accom- 

 panies the south-west monsoon.* 



* There are no data in Dr. Buist's report from which the diurnal variation in the force 

 of the wind may be judged of on the days during the south-west monsoon, when no al- 

 teration takes place in its direction. It would seem probable that on such days the vari- 

 ation should be a single progression, weakest towards daybreak, and strongest about 

 the hottest hour of the day. 



