Some simple devices to demonstrate important hydraulic principles are required in fluid mechanics or hydraulics classes. Those devices should be easy to make and operate with inexpensive cost. The writer has studied some simple demonstration apparatus of hydraulic principles which were made of cheap materials. An adjustable nozzle is one of them. It can be connected to an axial flow fan so as to use air flow. By operating this apparatus, pressure changes in gradually enlarged or contracted conduits could be observed.