348 NATURAL SCIENCE. Nov., 



clothing ; it is only occasionally that we lose much heat by conduction, 

 for instance, when we touch cold iron ; evaporation never entirely 

 ceases, and it varies in amount according to the needs of the body. 

 Man, in fact, is like one of the porous earthenware pots used in India 

 for cooling water. Put them in a hot, dry wind, and, the rate of 

 evaporation increasing, the water cools all the more rapidly, and the 

 Sahib's bath is ready all the sooner. Great heat can be endured if 

 only it is dry heat. A French physiologist once stopped a consider- 

 able time in a stove heated to 160 Fahr. ; but this is far below the 

 record. In 1760, according to the testimony of two French 

 academicians, a woman entered a stove heated to 237 Fahr. and 

 stayed there 12 minutes. Doctors Fordyce and Blayden were 

 able to remain in a chamber heated to 260 Fahr. I have been told 

 that a man who earned his living by feats of this kind found himself 

 compelled to rush precipitately from the heated oven because some 

 one, who was more scientific than kind, had placed a can of hot water 

 in one corner. Everyone knows how oppressive the heat of a hot-house 

 is. The heat of the vapour-baths in Russia is said sometimes to rise to 

 1 16 Fahr. ; but between this and 260 there is a great gulf. When the 

 air is moist evaporation is checked, and the human system has greater 

 difficulty in keeping its temperature down to the normal. 



Birds have, probably, nerves or nerve-fibres similar to our own 

 exercising a control over their temperature ; but evaporation from 

 the skin cannot go on except to an inconsiderable extent since they 

 have no sweat glands. Radiation on any large scale is prevented by 

 the thick covering of feathers ; but it is a question whether one 

 object of the bare patches, called apteria, may not be to allow of the 

 free access of air. Their temperature is very high, varying from 

 ioo° Fahr. to 112 . Every warm-blooded animal, we may take it for 

 granted, has an efficient heat-regulating system. The higher its 

 temperature the more efficient, we may assume, the system to be. 

 What, then, in birds takes the place of radiation from the skin ? 

 We may get a hint from watching a dog when taking vigorous 

 exercise. Dogs perspire through the tongue, and also, I believe, 

 through the feet, but not through the skin generally. The greatly - 

 increased rapidity of breathing is the chief means by which they keep 

 their temperature down. All the expired air is about the temperature 

 of the body. From this the cooling effect of rapid breathing may 

 easily be gathered ; and here we have, I believe, the explanation of 

 the great size of birds' air-sacs, and also, perhaps, of the long ramifi- 

 cations of the trachea which we find, for instance, in the crane. The 

 greater the amount of air breathed in and out, the more the body will 

 be cooled. In man and in birds the method is in reality the same ; 

 but in birds evaporation and radiation take place mainly from the 

 lungs and air-sacs, in man mainly from the skin. If we watch a bird's 

 breathing we obtain evidence of this ; when standing still it breathes 

 about 20 times in the minute, whereas an adult man in a sitting 



