"Entropy... we shall use this property in a specific and limited manner. ...The following are two implications of this property: 1. If a gas or vapor is compressed or expanded frictionlessly without adding or removing heat during the process, the entropy of the substance remains constant. 2. In the process implied in implication 1, the change in represents the amount of work per unit mass required by the compression or delivered by the expansion. Possibly the greatest possible use we shall have for entropy is to read lines of constant entropy on graphs in computing the work of compression in cycles."