Hydraulic macroeconomics is an informal characterization of certain types of macroeconomic study assuming aggregate social wealth (demand or supply) as somewhat smooth, constant and homogeneous. The term was first introduced as Hydraulic Keynesianism by Alan Coddington in classification of theoretical research methodologies in Keynesian Economics. Mainstream macroeconomics is "hydraulic." There is something called "aggregate demand" which you adjust by pumping in fiscal and monetary expansion.