of the Aqueous PortidnoftneAMospJieryi 4^ 



nine hnd two, or still later in the day, may have ascendjed and' 

 formed cloud, which cloud must have added to the general 

 vj'eight of the atmosphere. Had we accounts of the state of 

 the wet and dry-bulb thermometers, and of the dew-points at 

 different heights, there is little room to doubt that we might 

 trace the ascent of the vapour at Trevandrum until we found 

 it collected and floating in the atmosphere as a cloud, and in 

 that form adding to the general weight of the atmosphere. 



Colonel Sabine saj's that the maximum of vapour pressure 

 occurring at Trevandrum at 9 a.m. may be a consequence 

 of the sea-breeze blowing at that time. 1 have however 

 shown that the daily sea-breeze is itself produced by the di- 

 urnal cloud formation ; the sea-breeze is only another effect 

 arising from the same cause. The sea-breeze blows towards 

 the part, because the atmosphere has there been made lighter 

 than in adjoining parts by the heating power of condensing 

 vapour. The wind too that comes from the sea, particularly 

 in the fine season, when the diurnal disturbance of the baro- 

 meter is the greatest, comes more fully loaded with vapour 

 after nine o'clock than was the air over the land before that 

 time, and ought to increase the vapour pressure after that 

 hour, instead of stopping the increase. If ail the vapour 

 that arose had to come from the same land surface of the 

 locality, it might be supposed that evaporation could not con- 

 tinue to supply an adequate quantity to raise the dew-point 

 after nine; but when the sea-breeze sets in, a current of air 

 comes from an extensive sea surface, and brings with it the 

 vapour which had been evaporated from that surface, not only 

 up to nine o'clock, but until ten, twelve, or two o'clock, '0r' 

 still later ; the tendency of the sea-breeze is therefore not 't6' 

 reduce, but to increase the supply of vapour. It may also be 

 remarked, that whilst the maximum of vapour pressure is 

 said to occur at Trevandrum at nine o'clock, the sea-breeze 

 does not set in at Bombay until about eleven or half-past eleven. 

 Supposing both these places affected alike by the sea-breeze, 

 the cause of the stoppage of increase of vapour pressure, 

 whatever that cause may be, must have been in operation two 

 hours before the sea-breeze commenced blowing. odi 



Formation of cloud is a cause sufficiently powerful in its 

 operation to prevent the dew-point rising at Trevandrum after 

 9 A.M., as the vapour produced after that hour may be equal 

 only to that which is consumed in cloud formation ; and we 

 are authorised to conclude that it is to that formation we are 

 to attribute the stoppage of the dew-point at Plymouth at 

 eleven, and at Trevandrum at nine o'clock, instead of having 

 it rising with the temperature during the hottest portion ©f 



