CHAP, xxx.] INFLUENCE OF CLIMATE. 421 



the nest, but when removed from the nest, fell in one hour to 

 66 F., when the external air was at 62 F. ; and he showed that 

 this result was not to be attributed to their unfledged state. 



Young animals, therefore, require the aid of external warmth, 

 or at least of every means of retaining their own warmth. The 

 nests of birds not only serve to retain the warmth of the parent 

 during incubation, but also that of the young brood during their 

 tender age. The human infant stands no less in need of ex- 

 traneous warmth and it may be safely affirmed, that much of 

 the immense mortality of our infant population is owing to the 

 want of artificial heat. 



We must refer to what has been already said in the chapter on 

 food (Chapter XXII.), for observations on the relation of the kind 

 of food to the animal temperature. 



Influence of Climate and Seasons. The best account we possess of 

 the effect of climate results from 4000 experiments made on board 

 the Bonite, a French vessel, during her passage between Cape 

 Horn and the tropics. The mean temperature of 10 men at Cape 

 Horn with the thermometer at 32 F. was hardly two degrees lower 

 than at Calcutta, where the thermometer stood at 104 F. The 

 temperature of the body was found to rise and fall perceptibly, but 

 slightly to fall slowly in passing from a warm to a cold climate, 

 and to rise more rapidly in re-entering the torrid zone. 



The rate at which an animal loses its heat will depend both on 

 the coldness of the surrounding air, and on the rapidity of evapora- 

 tion. In temperate climates these causes differ inversely with the 

 season. In winter there is much loss by radiation, little by eva- 

 poration; in summer, the reverse. But Dr. Edwards has well shown 

 that these compensations are not the only cause of the uniformity 

 of temperature so wonderfully maintained, but that there is also in 

 animals a difference in the rate at which heat is produced, accord- 

 ing to the season, " the calorific faculty is more active in winter 

 than in summer." In winter there is a more active production 

 with greater loss, in summer a less production with smaller loss." 

 To test this, Dr. Edwards contrived a box, surrounded with ice, 

 having the same external temperature and the same humidity of 

 atmosphere in winter and summer, so that the loss by radiation 

 and evaporation should be the same in both cases. Into this ap- 

 paratus, in February, he placed 5 sparrows that had been living in 

 a warm room, and found that after an interval of 3 hours they 

 maintained their previous heat within 1 F. Again, in July, he 

 did the same with 4 others, and these after 3 hours were found to 



