Dr. Forry on the Climate of the United States, ^c. 223 



greater in excessive climes ; but as it supervenes upon a lower 

 winter temperature, the effect produced on the development of 

 vegetation is in an inverse ratio. The vernal increase of 30°-83, 

 for example, at Fort Snelling, comes upon a mean winter tem- 

 perature of 15° -95, while at Fort Sullivan, on the same parallel, 

 the increase of only 17°* 16 follows a winter temperature as high 

 as 22°-95. Between northern and southern latitudes, this con- 

 trast is still more marked ; for, while at Fort Snelling there is a 

 difference of 13°'46 between the months of February and March, 

 and at Key West only l°-56, the temperature of February at the 

 former is 18°-66 and at the latter 72°-15. 



What month expresses the tiearest equivalent to the mean an- 

 nual temperature ? This is a question which has excited con- 

 siderable controversy, and in regard to which there still exists an 

 opposite diversity of opinion. By Kirwan, it is strenuously con- 

 tended that the month of April expresses this equivalent ; while 

 Humboldt, on the other hand, shows by extensive tabular state- 

 ments that October IS better entitled to this characteristic : and 

 on either side of this question are arranged many other authors 

 of lesser note. As th6 laws of nature are universal, these phe- 

 nomena, like all others, must be susceptible of systematic arrange- 

 ment ; and lest it may be thought presumptuous in the author to 

 attempt to decide between such high authorities, he will state in 

 advance that the diverse systems of climate presented in the nor- 

 thern regions of the United States, on the same parallels, afford a 

 means of comparison doubtless heretofore unequalled. 



As respects the controverted question, whether April or Octo- 

 ber expresses a nearer equivalent to the mean annual temperature, 

 the following deduction is clearly authorized by the tabular ab- 

 stracts appended to the writer's work on "The Climate of the 

 United States, etc." — In excessive climates, the mean tempera- 

 ture of Aprilis generally as high as that of the year, while that 

 of October is considerably higher ; and in modified climes, it 

 will be found that the former is generally as much lower as the 

 latter is higher. Now this relation is precisely what might have 

 been anticipated from a consideration of the preceding facts ; for 

 as the vernal increase of temperature is always much greater in 

 excessive than in modified climates, it follows that, if under any 

 circumstances, April expresses a nearer equivalent than October, 

 it must be when its mean temperature is augmented by a sudden 

 vernal increase. 



