The New Student's Reference Work/Isothermal Line

Isothermal (ī′ sṓ-tẽr′ mal) Line or Isotherm, a term used in meteorology to denote the line obtained by joining all contiguous points having the same temperature. The United States Weather Bureau publishes each month a map of the United States on which are drawn lines connecting adjoining localities which have had the same mean temperature for the past month. These lines are called isotherms. In physics the word is used in a slightly enlarged sense to indicate any curve which represents the manner in which one quantity varies with another while the temperature remains constant. Thus the curve which shows how the volume of any given mass of gas changes, when the pressure is altered, is called an isothermal line.  For full discussion see Maxwell’s Theory of Heat, chap. vi.