1871.] Proceedings of the Asiatic Society. 45 
the absolute maximum. But with advance into the interior of the 
continent, the phenomena change. For instance at St. Petersburgh, 
the summer maximum divides itself into two subordinate maxima. 
Further on, as at Moscow, these two relative maxima are still 
found, but their absolute magnitudes are diminished and the sink- 
ing between them increased ; and finally on this side of the Ural 
the summer maximum disappears altogether. 'The explanation 
which is commonly given, is that which I first referred to in ac- 
counting for the diurnal oscillations of the barometer. It is argued 
with much force that the rise towards a maximum goes on as long 
as the additions of vapour which are lifted up by the action of the 
heated surface continue to be more than sufficient to compensate 
for the increase of rarifaction brought about by the same agency. 
But as soon as the supply for any reason which may locally obtain 
falls below this amount, the process of rarifaction prevails to di- 
minish the material in the atmospheric column and consequently 
to lower the pressure. Thus it would happen that all places which 
are in this way affected by, so to speak, a deficiency in the supply 
of vapour are surrounded by places where the atmospheric pres- 
sure at the same time stands relatively at a maximum. ‘There is 
an element, which I have not yet mentioned, and which is more 
than any other influential as a cause affecting the efficiency of the 
earth’s surface as a heating agent, and therefore affecting the den- 
sity of the superincumbent atmospheric column. I refer to the 
activity of terrestrial radiation. This not only depends upon the ma- 
terial condition of the surface itself, but also upon the circumstances 
of the local situation. We all know the striking difference in this 
respect between the plains and a hill station. Professor Tyndall 
is of opinion that the presence of invisible vapour in the air oper- 
ates to check the radiation from the earth’s surface, and so is a 
principal ingredient in the varying circumstances upon which sur- 
face temperature depends. 
That loci of maximum and minimum pressure do periodically 
manifest themselves as a gonsequence of the recurrence of the same 
local conditions is certain, as also that periodic winds or modifica- 
tions of winds are the result. I will repeat that we seem to have 
especial advantages in this country for working out the problem 
