1873.] -L^J [Blodget. 



racy. So also of the averages or fixed constants of heat ; there is little 

 difficulty in reducing all the irregularities disclosed in successive months, 

 seasons or years to determinate values, fixing the isothermal illustration 

 with reasonable precision. 



But the great single non-periodic variations of heat are more difficult 

 to grapple with than any other phenomena of their class. They are not 

 traceable, as storms are, from any point of natural initiation, through a 

 moving path of growth to maximum proportions, followed by regnal 

 and natural exhaustion until the equilibrium is restored. An excess of 

 moisture arising in the atmospheric circulation at a colder latitude must 

 be precipitated from the air which can no longer sustain it ; and in the 

 course of this natural and inevitable condensation, all the known phe- 

 nomena of storms are developed. But there are changes of the measure 

 of heat which occur in quite as large a measure wholly independently of 

 any such phenomena, as they do in concurrence with them. Indeed, there 

 are at times extraordinary depressions of temperature departing widely 

 from the normal measures for the month or season in which they occur, 

 which appear to strike down — not to be transferred along the surface with 

 any atmospheric movement — and to force from the air every vestige of 

 contained moisture ; as if some external compressing force had been 

 applied. 



As I have proposed only to open the subject at the present time, I beg 

 to offer a few propositions for consideration rather than to suggest that 

 they are even preliminarily proved. It is a very laborious work to inves- 

 tigate the subject through the rigid processes of induction which can alone 

 establish positive principles. It is easier to indicate some negative deter- 

 minations ; and these are so important, and so much at variance with the 

 analogies of change in the atmospheric problems heretofore investigated, 

 as to induce me to anticipate more conclusive results and present them in 

 this paper. The following are the propositions ; 



1. The greater non-periodic changes in the measure of heat in the tem- 

 perate climates of the North American Continent are not connected with 

 or dependant upon the system of atmospheric circulation of these lati- 

 tudes ; nor on the incidents of this system, in the evaporation and precipi- 

 tation of moisture, nor on the greater phenomena of such precipitation in 

 the form of local or general storms. 



2. There is no evidence of movement in these greater changes, either 

 with or against the course of movement belonging to the ponderable ele- 

 ments of the atmosphere. 



3. The periods of extreme cold do not come from, or connect with like 

 conditions at the North ; they are not transferred from the North South- 

 ward ; often the reverse is the case, and unusual mildness prevails in 

 observed portions of British America, at the time that almost Arctic 

 severity prevails in much lower latitudes and in various parts of the 

 Northern and Central States. 



4. The periods of extreme heat do not come from, or connect with 

 periods of extreme heat at the South. 



