INDIAN SUMMER. 139 



ispliere, in which the gain is greater than the loss, the consequent con- 

 traction in bulk and diminution in height of its portion of the atmos- 

 phere, must be followed by a determination of the upper air towards the 

 North, and of the lower towards the South, thus producing a slow cir- 

 culation between the two hemispheres. Hence the greater prevalence 

 of northerly winds during this than during any other portion of the 

 year in our latitude. During May, June, and July, the same causes, in 

 a reverse order, must produce a determination of the under portions of 

 the atmosphere towards the North ; and hence the greater prevalence 

 of southerly winds during these months, than during any other part of 

 the year. 



Now, immediately after the minimum temperature has been attained 

 in the northern, and the maximum in the southern hemisphere, the 

 southward determination, or the great southern oscillation will be check- 

 ed, and soon begin to move in an opposite direction, by the reversal of 

 the thermal action ; there being now a slight gain of temperature in the 

 north and loss in the south. But, before this great oscillation can move 

 in a contrary direction, there must be a temporary equilibrium. Ac- 

 cordingly, near the close of January, a season of calm, mild and pleas- 

 ant weather, is almost invariably enjoyed; and the month of August is 

 remarkable for its quiet and sultry weather. That the smoke of Janu- 

 ary and August is less dense than that of April and October, is probably 

 due to the fact that the equilibrium of pressure between the two hemis- 

 pheres, is kept in a very unstable condition by the want of an equili- 

 brium at those times between land and ocean. 



But the circulation preceding and succeeding these four seasons of 

 comparative equilibrium, is not regular and continuous. It is frequently 

 interrupted by periods of rest, producing the alternations of the weather 

 which are found to exist. Thus, for example, the remarkably fine 

 weather, almost invariably enjoyed about the 20th of February, may be 

 but an interrupted stage of the great annual wave which has then com- 

 menced to flow towards the north. 



But we are reminded that it is time, without further enumerations, to 

 close our remarks upon the " Indian Summer," and other affiliated phe- 

 nomena; hoping that tlie explanation which we have offered, if it do 

 not commend itself to the approval of those who have given it their at- 

 tention, will at least excite them to the effort of discovering and offer- 

 ing one which may prove satisfactory. 



