EDITOKIAL. 603 



temperature between the two linings, which difference can then be 

 equalized by supplying heat through the electric wires or cold 

 through the water pipes in the air space just outside the zinc wall. 

 In this manner a balance is readily maintained which checks the 

 escape of heat and makes its accurate measurement possible; and in 

 actual practice little change is needed after the apparatus has been 

 adjusted. 



Heat given off by the subject is determined by means of a current 

 of water passing through a heat-absorbing device within the chamber. 

 The rise in temperature of the water in its passage through the 

 absorber multiplied by the weight of water gives the number of 

 calories of heat produced. 



Formerly this measurement of the heat was effected by reading and 

 recording two mercury thermometers every two minutes throughout 

 the experiment, one thermometer showing the temperature of the 

 water as it entered the heat absorbers and the other as it left. This 

 was a confining and tedious operation, subject to error due to the 

 individual factor in making the readings. Such errors could not be 

 detected, and in experiments running continuously for several days 

 it was difficult for even the most careful observers to guard against 

 them. 



In the new apparatus the measurement of heat is accomplished by 

 means of two ingenious automatic electrical devices operating quite 

 independently of the observer. One of these regulates the tempera- 

 ture of the ingoing water and the other measures and records the 

 temperature difference in the water before and after passing the heat 

 absorber. These devices greatly reduce the labor of attendance and 

 eliminate a possibility of serious error. Together they constitute 

 the most important recent improvement made in the apparatus. The 

 accuracy and reliability with which they operate throughout a long 

 experiment is a triumph of electrical skill. 



The regulating device brings the temperature of the water enter- 

 ing the heat absorbers entirel}'^ under control and renders it uniform 

 after the device has been set for the desired temperature. The water 

 flowing toward the heat absorber is first cooled to a temperature 

 below that desired, in order to bring it under control ; and at a point 

 in the circuit just outside the chamber it passes through a narrow 

 channel, where it is heated by a current of electricity passing through 

 a resistance wire. It flows next through a small mixing device to 

 equalize its temperature, and then through another small channel in 

 which it surrounds an electric resistance thermometer. This ther- 

 mometer forms one arm of a ^Vlieatstone bridge. The galvanometer 

 in the bridge circuit is connected with an apparatus governing the 

 action of a small motor, which actuates a rheostat so as to increase 

 or decrease the quantity of current in the heater. 



