70 Proceedings of the Asiatie Society. [ Marcn, 
tained over the laws of the ordinary diurnal change of temperature, 
and to the study of these, he would express a hope, that Indian 
observers would apply themselves. The primary causes of these 
changes were simple enough. On the one side, the Sun during the 
day added to the heat of the air and the earth, and on the other 
the air and the earth during the night threw off their heat into 
celestial space. Very little was yet known of how these operations 
took place, or why it was that special laws of increase and decrease 
of temperature governed each season or each locality. One of the 
causes of such variations he might refer to, (as before, rather in 
illustration of the general scope of his advice, than as an attempt 
to deal exhaustively with the subject), was the quantity of vapour 
in the air, by reason of which its power of transmitting radiant 
heat varied. As the air was dry, it transmitted more; as it was 
filled with vapour, it transmitted less heat. ‘Thus the diurnal va- 
riations both by day and night would increase in extent as the air 
was drier, and vice versa. Col. Strachey had examined the Madras 
observations with a view of ascertaining how the matter was, after 
the suggestion had been made by the researches of Professor Tyn- 
dall, and the result, as above stated, quite corroborated the labora- 
tory experiments. 
The diurnal winds of Upper India were very well known to all 
persons acquainted with that part of the country. That they were 
due to the daily variation of the pressure he had little doubt. They 
were not confined to India at all, and in truth extended all over 
Southern Asia up to the Caspian. ‘The correctness of this theoreti- 
cal explanation of these diurnal westerly winds, was, he thought, 
quite confirmed by the circumstance that during the months of dry 
westerly wind a faint easterly wind was common early in the 
morning, showing that the high pressure to the east of the place 
of observation had a similar effect to that produced to the west of 
it. Of course as the actual course of the crest of the wave of pres- 
sure was east to west, and the great fall of pressure was to the 
east of the crest, the westerly wind must be the best marked. 
It is important, Col. 8. said, always to bear in mind that wind is 
nothing more than a consequence of inequality of pressure, and, 
therefore, commonly, if not always, more or less directly of changes 
