828 APPLICATION OF PRINCIPLES. 



thou<_>h not to the same amount as they purify it dur- 

 ing the day. It is, therefore, most important that the 

 temperature of glass houses should, under all circum- 

 stances whatever, be lower than that of the day ; and 

 it is probable that this ought to take place to a greater 

 extent than is generally imagined by even the best 

 practical gardeners. We are told that, in Jamaica 

 and other mountainous islands of the West Indies, 

 the air upon the mountains becomes, soon after sun- 

 set, chilled and condensed, and, in consequence of its 

 superior gravity, descends and displaces the warm air 

 of the valleys ; yet the sugar-canes are so far from be- 

 ing injured by this decrease of temperature, that the 

 sugars of Jamaica take a higher price in the market 

 than those of the less elevated islands, of which the 

 temperature of the daj' and night is subject to much 

 less alteration. At Fattehpur, in the East Indies, the 

 difference in temperature between night and day 

 amounts to as much as 78°, on an average of the 

 whole year ; in April the greatest heat by day is 110°, 

 that of night is only 65° ; in January the thermometer 

 falls to 38° at night, while the day is 76' ; and there 

 are 40 degrees of difference between the day andnight 

 in May, one of the_ hottest months, when the thermo- 

 meter ranges as high as 115°. At Calcutta, in May, 

 the thermometer averages 93° in the day, and 79° at 

 sunrise ; while in January the temperatures are 77° 

 and 56° respectively, for those two peridds. 



When we compare these facts with the habits of 

 plants just adverted to, we must, I think, see that it 

 is the purpose of nature to reduce the force which 



