the first model studied employed a long horizontal glass tube , 4 mm in diameter , down which water flowed under a constant pressure head . A hypodermic needle (No.22 gauge) was inserted through the wall of a plastic tube at the entrance . The needle was connected through narrow "thermometer tubing" to a reservoir of air under pressure , so that it emerged into the stream at a pressure close to that of the water . It was found that an oscillatory condition could be set up , in which bubbles of air entered the stream rhythmically , forming a train of "boluses" . Adjustment of the relative pressure allowed bubbles of different and spacing to be obtained.