62 PHYSIOLOGY CHAP. 



of a balance, which carries a pen at one end. It is obvious that if the two 

 systems be heated to the same degree, the two cylinders will not be displaced 

 and the balance will remain in equilibrium. If, however, one of the calori- 

 meters be heated while the temperature of the other remains unaltered, the 

 expansion of the air in the first will raise the corresponding cylinder, and the 

 pen will trace an ascending line on the kymograph. The arrangement of 



Fio. 11. D'Arsonval's differential calorimeter for animals. 



this differentiarcalorimeter has also the advantage of allowing comparative 

 experiments to be carried out on two animals at the same time. 



The following table shows the numerous, data obtained by 

 Atwater in a series of experiments on man. In it the calories 

 estimated (A) are compared with those found with the calorimeter 

 (B), and the percentage difference (B-A) is given. All the results 

 refer to the same person, and the figures express in large calories 

 the heat produced for an average of twenty - four hours on a 

 normal mixed diet during rest and muscular work. 



[TABLE 



