CHAP, in.] ON THE CONDUCTimilTY OF HEAT. 359 



badly conducting layer. And then a roof covered with liaulm and 

 turf, and plantations of trees whose shade protects the ice-house from 

 the rays of the sun, complete the process by which the interior of the 

 cavity is rendered altogether impermeable to the external heat. 



All these precautions are based, as we see, on the feeble con- 

 ductivity for heat of earth, bricks, and loosely packed materials. 



II. CLOTHES. 



From houses let us turn to clothes. 



There are but few countries where clothes are not indispensable for 

 protection against the weather, and especially against excesses of 

 temperature, either in winter, or in summer. Man is not naturally 

 protected against these excesses, and he may suffer greatly from them. 

 He is not, as most animals are, provided with a covering of hair, or 

 feathers, or down, or with a fleece more or less thick to protect him 

 from the inclemency of the air, and he must have recourse to his in- 

 dustry which can do no more than imitate nature in a more or less 

 intelligent manner, according to the degree of civilisation he has 

 attained to. 



In the state of barbarism which characterised the first ages of 

 mankind, a state of which we have still many remains, men could do 

 no better than cover themselves with the skins of the animals they 

 killed in the chase. This primitive garment is still that of many 

 barbarous tribes. In polar climates the Esquimaux, the Lapps, the 

 Siberians, clothe themselves in skins of bears and reindeers, which 

 they cut in a rough sort of fashion. The hide is impermeable to 

 moisture, but it is the covering of hair which forms the real protective 

 layer against the cold, on account of its feeble conductivity for heat. 



The temperature of the human body is pretty nearly constant in 

 all climates and through all seasons. It is not modified by action 

 from the outside, so to speak. " The blood of the Laplander," says 

 Tyndall, 1 "is sensibly as warm as that of the Hindoo, while an 

 Englishman in sailing from the north pole to the south finds his blood 

 temperature hnrdly heightened by hisapprcach to the equator, and hardly 

 diminished by his approach to the antarctic pole." What forces us 



1 Heat as a Mode of Motion. 



