with Geographic Botany, — Temperature, 177 



how far this obtains in the tropics, and a further decrease 

 must be allowed for nocturnal radiation. Mr. Daniel has 

 made us acquainted with the unexpected circumstance, that 

 in our own climate, vegetation, in ten months of the year, is 

 subject to a temperature below the freezing point. Even in 

 the two months which are exceptions, July and August, the 

 thermometer exposed to terrestrial radiation sometimes sinks 

 to 35°. 



Cultivation has been always allowed to have a material in- 

 fluence on the temperature, but as people formerly were much 

 less curious on the subject than at present, it is almost an 

 impossibility to obtain correct data. Even the few years that 

 have passed, since the reclaiming of the land in many of our 

 colonies, had their commencement at a period when registers 

 of daily fluctuations in the thermometer were overlooked, 

 among the claims of more important pursuits. The effects 

 must of course vary according to the circumstances of a tract 

 of country previous to its being submitted to cultivation. 

 Deep impervious forests are frequently removed by the dili- 

 gence of the settler, and a new and perhaps extensive surface 

 exposed to the direct action of the sun and air. The changes 

 which follow here must be very different from those occurring 

 over another space of country, where cultivation, instead of 

 circumscribing the reign of the vegetable kingdom, continually 

 adds to it. 



To place these opposite conditions in a clearer light, we will 

 select two instances, and by detailing the chief peculiarities of 

 the climate of each, may be enabled to draw a satisfactory com- 

 parison between them. The island of Ascension is almost 

 destitute of vegetation ; in the sheltered ravines and tempo- 

 rary water-courses of the rainy season, a few ferns and other 

 plants thrive. Their number is small, and the soil almost 

 everywhere without a flora. The climate, however, is delight- 

 ful ; it is rather warm, b" 4 : very healthy. In the hot season 

 the thermometer only ranges ten degrees in the twenty-four 

 hours, and during the time of the rains only eight. The 

 whole range of the year is not more than sixteen or eighteen 

 degrees. Rain is scarce even in the proper season, a circum- 

 stance always occurring in similar situations, and dews are 

 also far from frequent; indeed the small depression of the 

 temperature during the night can seldom be expected to reach 

 the point of deposition. Very different conditions will be 

 found in that part of North America which comprehends the 

 Canadas and the northern portion of the United States. This 

 is truly a climate of extremes ; the winter is intensely cold 

 and protracted, snow lying long on the ground ; when sum- 



