ANIMAL HEAT. 593 



pation is greater when the animal is in the water than when exposed to the air 

 if both water and air be of the same temperature, because water is a better 

 conductor of heat and consequently withdraws heat from the body more 

 rapidly. The higher the temperature of the surroundings the higher the 

 bodily temperature of cold-blooded animals, consequently the greater are heat- 

 production and heat-dissipation. In warm-blooded animals the effect on both 

 heat-production and heat-dissipation is in inverse relation to the surrounding 

 temperature (unless the bodily temperature is affected), external heat decreasing 

 both heat-dissipation and heat-production, and internal heat increasing both. 



Subcutaneous fat is a poor conductor of heat, consequently the greater the 

 abundance of it the greater the hindrance offered to the dissipation of heat. 

 The value of fat in this respect is illustrated in water-fowl, which, as a rule, 

 are far more abundantly supplied with fat than other species ; and by the ex- 

 ceptional abundance of subcutaneous fat in species of fowl which inhabit very 

 cold waters. Bathing the skin with grease hinders radiation, and is adopted 

 by swimmers both to conserve the bodily heat and to protect the skin. 



When air and water are of the same temperature, heat-dissipation is greater 

 when the animal is exposed to the water, because the latter is a better con- 

 ductor. Heat-loss is greater in dry than in moist air, other things being 

 equal, because in the former the evaporation of sweat from the body and the 

 loss of water from the lungs are favored, the vaporization of water affecting 

 heat-dissipation more decidedly than the moisture of the air. Heat-dissipation 

 is more active in cold moist air than in cold dry air. Cold air is not favorable 

 to the vaporization of water, whereas cold moist air has a higher specifich^at 

 than the dry air, and thus tends to carry off heat more rapid ly~f 



The character of the covering of the body is of great importance. This 

 is illustrated in the changes which occur in the natural covering of animals 

 during warm and cold seasons, and in the characters of the fur of species 

 which inhabit very cold or very warm climates. During the winter the fur 

 is longer and thicker than during the summer. Animals living in cold or hot 

 climates are supplied with a relatively greater or less abundance of fur or 

 feathers and subcutaneous fat. Man provides for changes of the seasons by 

 modifying the quantity and quality of his clothing. In the adaptation of 

 dress to climate, the conductivity, radiating coefficient, hygroscopic capacity, 

 porosity, weight, and color of the clothing are important factors. The poorest 

 conductors, other things being equal, make the warmest clothing; fur and 

 wool are poor conductors and therefore are adapted especially for cold seasons 

 and climates, while cotton and linen are good conductors and therefore make 

 cool clothing. The radiating coefficient depends upon the conductivity of the 

 material and the character of the radiating surface. The coarser the material 

 the better the radiating surface, hence the better the conductor and the cooler 

 the clothing. The hygroscopic character of the clothing is of far more 

 importance than is generally believed. Articles of clothing having a large 

 capacity for absorbing and retaining moisture are, other things being equal, 

 of more value, especially for underwear, than those possessing the opposite 



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