140 ON ATMOSPHERIC AND BAROMETRIC 



pressure without an increase of temperature. It 

 is, then, the reduced pressure of vapour, and not 

 any diminution of gaseous pressure which we 

 presume causes the barometer to fall during the 

 time last named. The land wind blows at the 

 same time ; because in the absence of the sun the 

 gaseous part of the atmosphere continues to cool 

 over the land, and the land gases press on the 

 lighter sea gases, and flow from the land towards 

 the sea, constituting the land wind, which in- 

 creases in strength with the cooling of the gases 

 over the land up to about sunrise. 



Here, then, we see why — when, from the in- 

 creasing strength of the land wind, we should 

 expect that the barometer would rise — it falls ! 

 We see that the fall of that instrument is a con- 

 sequence of reduced vapour pressure alone, whilst 

 the reduction of that pressure does not prevent 

 the colder gases over the land from flowing as a 

 land wind towards the sea, where the atmospheric 

 gases are warmer. 



The approach of the morning sun prevents fur- 

 ther cooling, and his rise soon begins to warm the 

 atmosphere ; the land breeze then diminishes in 

 strength, until about ten o'clock when it ceases. 



