APPENDIX 363 



conditions, (i) when the heat developed by the com- 

 pression is allowed to escape through the walls of the 

 vessel to the outside, or when the heat lost in the ex- 

 pansion of the gas is compensated by the conduction 

 of heat through the walls of the vessel from outside ; 

 and (2) when the heat developed is retained in the gas, 

 as when the latter is contained in a vessel the walls of 

 which do not conduct heat. The pressure of the gas is 

 measured along the horizontal axis, and the volume is 

 measured along the vertical axis, and a curve is drawn 

 so that for any value of the pressure there is a cor- 

 responding value of the volume. Thus the values of 

 the pressures p and p l in the diagram correspond to the 

 value of the volume v. The curve relating the change 

 of pressure with a corresponding change of volume is, 

 in general, that called a rectangular hyperbola. But 

 there are two kinds of such curves : (i) that which we 

 obtain by plotting the corresponding values of pressure 

 and volume, when the temperature of the gas remains 

 constant throughout the series of changes, that is, 

 when the rise of temperature which would occur 

 when the gas is compressed is compensated by the 

 conduction of this heat to the outside of the vessel 

 containing the gas. Such a series of changes of pres- 

 sure and volume is called an isothermal one. (2) When 

 the heat developed by the compression of the gas is 

 retained in the gas, as when the walls of the vessel in 

 which these changes are effected are such as do not 

 conduct heat : such a series of changes is called an 

 adiabatic one. Adiabatic curves are steeper than are 

 isothermal ones. 



THE CARNOT ENGINE 



This is an imaginary mechanism which performs a 

 certain cycle of operations. It does not really exist, 



