EVOLUTION OF HEAT. 



617 



seem, like infants and young children, to have less power of resisting external 

 cold, the temperature of their bodies being more easily and considerably re- 

 duced by it than is that of adults; and hence probably it has happened, that 

 popular opinion assigns to them an habitually inferior temperature. 



b. A slight diurnal variation in the temperature of the body appears usually 

 to take place, quite irrespectively of external heat or cold; but this does not 

 seem to be very constant either in its period or its degree, and is seldom very 

 considerable. Thus Dr. Davy found, from a long series of observations carried 

 on upon himself whilst in England, that the body was warmest in the morning, 

 and coldest at night ; whilst the reverse was the case in Barbadoes. The follow- 

 ing table gives his average results : 



From the observations of M. Chossat on Birds, in which <fche diurnal variation 

 amounts to li Fahr., it seems that the maximum is pretfy constantly at noon, 

 and the minimum near midnight; and this corresponds well with what has 

 already been pointed out, with regard to the relative activity of respiration at 

 different periods of the twenty-four hours ( 564, i). Probably there is a less 

 capacity for generating heat during the night ; so that, if the body be insuffi- 

 ciently protected by clothing, or be exposed to a low degree of external tempera- 

 ture, its own temperature will be more readily lowered : and thus the minimum 

 of the whole day may come to present itself at this part of it, in a temperate 

 climate; whilst in a tropical climate, the light bed-covering and free circulation 

 of air usual in the sleeping-room, together with the depressing influence of re- 

 pose, would tend to render the early morning temperature the lowest. 



c. That an increase in the heat of the body is produced by exercise, and that 

 repose tends to its reduction, is a matter of familiar experience; but the observa- 

 tions of Dr. Davy show that there is scarcely any perceptible difference in the 

 heat of the deep-seated parts, the augmentation and depression being confined 

 to the extremities. Thus, on one occasion recorded by him, the temperature of 

 the air of the room before walking being 60, that of the feet (the thermometer 

 being placed between the toes) being only 66, that of the thermometer under 

 the tongue being 98, and that of the urine 100 the temperature after a walk 

 in the open air at 40, the exercise having diffused a feeling of gentle warmth 

 through the body, was 96.5 in the feet, 97 in the hands, 98 under the tongue, 

 and 101 in the urine. So on another occasion, the temperature having been 

 66 in the room, 75 in the feet, 81 in the hands, 98 under the tongue, and 

 100 in the urine after a walk in air at 50, the temperature was 99 in the 

 feet, 98 in the hands, 98 under the tongue, and 101.5 in the urine. 



d. The influence of ingestion of food upon the temperature of the body has 

 not yet been duly investigated. Common experience leads to the conclusion, 

 that after a meal, as after exercise, there is a greater warmth in the extremities ; 

 but Dr. Davy's observations show that, in his own case, whilst in England, 

 there was usually an appreciable depression immediately after dinner, though 

 in Barbadoes the effect of a moderate meal was to produce an elevation. In 

 both cases, however, Dr. D. observed that the ingestion of wine has a positive- 

 ly depressing influence on the temperature of the body, which increases with 

 the quantity taken ; and it may have been the constant employment of wine 



