282 THE HUMAN BODY. 



is the number of kilogrammeters or foot-pounds of work its 

 unit quantity would perform, if converted into mechanical 

 work and used to raise a weight. For example the unit 

 quantity of heat is that necessary to raise one kilogram of 

 water one degree centigrade in temperature; or sometimes, 

 in books written in English, the quantity necessary to warm 

 one pound of water one degree Fahrenheit. When there- 

 fore we say that the mechanical equivalent of heat is 423 

 kilogrammeters we mean that the quantity of heat which 

 would raise one kilogram of water in temperature from 

 4 C. to 5 C. would, if all turned into mechanical work, 

 be able to raise one kilogram 423 meters against the attrac- 

 tion of the earth; and conversely that this amount of me- 

 chanical work if turned into heat would warm a kilogram 

 of water one degree centigrade. The mechanical equiva- 

 lent of heat, taking the Fahrenheit thermometric scale and 

 using feet and pounds as measures, is 772 foot-pounds. 



Potential and Kinetic Energy. At times energy seems 

 to be lost. Ordinarily we only observe it when it is doing 

 work and producing some change in matter : but sometimes 

 it is at rest, stored away and producing no changes that 

 we recognize and thus seems to have been destroyed. 

 Energy at work is known as Tcinetic energy; energy at rest, 

 not producing changes in matter, is called potential energy. 

 Suppose a stone pulled up by a string and left suspended 

 in the air. We know a certain amount of energy was 

 used to lift it; but while it hangs we have neither heat nor 

 light nor mechanical work to represent it. Still the energy 

 is not lost; we know we have only to cut the string and 

 the weight will fall, and striking something give rise to 

 heat. Or we may wind up a spring and keep it so by a 

 catch. In winding it up a certain amount of energy in 

 the form of mechanical work was used to alter the form 

 of the spring. Until the catch is removed this energy re- 

 mains stored away as potential energy: but we know it is 

 not lost. Once the spring is let loose again it may drive a 

 clock or a watch, and in so doing will perform again just 

 so much work as was spent in coiling it; and when the 

 watch has run down this energy will all have been turned 



