362 Dr. W. Marcet and Mr. K. B. Moris. The Efficiency 



would probably happen that if a person could climb at the rate of 

 522 m. per hour, as Smith did on the treadmill, then he would expire 

 about five times as much C0 2 as in the state of repose ; there is no 

 reference to the oxygen absorbed in addition to the carbonic acid 

 expired. From these considerations it will be understood how Helm- 

 holtz obtained the figure accepted in a general way as the value of the 

 efficiency of the human body, and which is close to the figure we have 

 found. 



In order to determine experimentally the heat given out under a 

 definite exercise, and ascertain that utilised in the work done, a brake 

 was used, being a modification of that known to engineers as Pronie's. 

 This brake (see diagram) consists of an iron fly-wheel resting on an 

 iron stand and worked by a handle projecting inside the calorimeter 

 through a stuffing box ; a counter registers the number of revolutions 

 throughout the experiment. The wheel is brought into contact with a 

 semicircle of hard wood in sections fastened to a strap ; by tightening 

 this strap the semicircle of wood can be pressed more or less against 

 the wheel, and thus the friction can be regulated. 



On turning the wheel pressing against the wooden blocks, the 

 tendency would be to give the blocks a revolving motion, but the force 

 applied, instead of carrying them round and round, causes the wheel to 

 slip over their surface, and in doing so to exert a degree of friction 

 sufficient to raise a lever weighted at the end, and maintain it in a 

 horizontal position as long as the wheel is rotated. 



