66 Proceedings of the Asiatic Society. [ Marcy 
by a hygrometer, from the total pressure, indicated by the barome- 
ter, was to commit an act of folly. It meant nothing. It repre- 
sented no physical fact. The vapour tension at the earth’s surface 
was not the result of the pressure of the particles of vapour in the 
upper strata of the atmosphere, but of something quite different. 
It was the measure of the resistance offered to the passage of the 
vapour particles in an upward direction by the air particles, and 
the superincumbent vapour particles together ; and the condition of 
the vapour in the upper strata proved that this resistance of the 
air particles was very great, so that, roughly speaking, the vapour 
tension was commonly about four times as great at any place as the 
pressure from above of the vapour particles. Of course there were 
great local variations from any such rough general average, but 
the average might be mentioned to show how senseless was the 
subtraction of the observed vapour tension from the observed total 
pressure. 
Referring next to the local variations of vapour pressure at Cal- 
cutta, Col. Strachey remarked, that the only satisfactory way of con- 
sidering such phenomena was in connexion with their physical 
causes, and that most of the apparent peculiarities, such as those 
noticed by Mr. Blanford, might readily be explained when viewed 
in this manner. He (Col. 8.) had not had the means of critically 
examining the variations of vapour tension at Calcutta, but he re- 
membered enough of the results of such an examination made by 
him of the phenomena at Madras, to be able to indicate to the So- 
ciety the kind of analysis of these facts that he had suggested. Thus 
it was observed at Madras that at a certain hotr of the day a very 
sudden increase occurred in the quantity of vapour. This was at 
once explained by the fact that at this hour the sea breeze became 
established. As the heat of the day increased, the wind blowing 
from over the sea brought in more vapour, and a maximum occurred 
in the afternoon. As the temperature fell, and the wind veered 
landward, the vapour became less, and when the land wind was 
thoroughly established, the vapour became much less, and a mini- 
mum was arrived at somewhere near the minimum of temperature, 
But certain subsidiary complications of this general rule were also 
observable. After the sun rose, the heat, radiated to the earth, 
