30 Lieut.-Col. Sabine on some. Points in 



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 the columns of air 

 which rest respectively on the surfaces 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 tem- 

 perature is most frequently insufficient to produce that alternation 

 in the direction of the wind, which prevails uninterruptedly during 

 the larger portion. 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 alternation of land and sea breezes was percep- 

 tible. The causes which produce the alternation are not therefore 

 wholly inoperative, though the effects are comparatively feeble during 

 the clouded weather which accompanies the south-west monsoon*. 



If we now view together the diurnal variations of the wind and 

 gaseous pressure, as shown in Plate I., we find a minimum of pres- 

 sure coinciding with the greatest strength of the sea breeze ; a 

 second minimum of pressure coinciding with the greatest strength 

 of the land breeze ; and a maximum of pressure at each of the pe- 

 riods when a change takes place in the direction of the aerial cur- 

 rents ; or, otherwise stated, we find a decrease of pressure coincident 

 with the increase of strength both of the land and sea breezes, and 

 an increase of pressure coincident with their decline in strength. 



The facts thus stated appear to me to admit of the following ex- 

 planation : — the diminution of pressure which precedes the mini- 

 mum at 4 h is occasioned by the rarefaction and ascent of the co- 

 lumn during the heat of the day, and its consequent overflow in the 

 higher regions of the atmosphere, which is but partially counter- 

 balanced in the forenoon by the influx of the sea breeze at the lower 

 part of the column. Shortly after the hottest hour is passed, the 

 overflow above and the supply below become equal in amount, and 

 the diminution of pressure ceases. As the temperature falls to- 

 wards evening, the column progressively contracts, when the influx 

 from the sea breeze more than counterbalances the overflow, and 

 the pressure again increases until a temporary equilibrium is re- 

 stored, when the sea breeze ceases and the pressure is stationary. 



As the night advances, the air over the land becomes colder than 

 over the sea ; the length of the column over the land contracts, and 

 the air in its lower part becomes denser than in that over the sea : 

 an interchange then commences of an opposite character to that 

 which prevailed during the day. The outward flow is now from 

 the lower part of the column, or from the land towards the sea, 



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

 the force of the wind may be judged of in the days during the south-west mon- 

 soon, when no alternation takes place in its direction. It would seem probable 

 that on such days the variation should be a single progression, weakest towards 

 daybreak, and strongest about the hottest hour of the day. 



