  Many experimenters, starting with Ernst Mach in 1883, have reported that if a
device alternately sucks in and then expels a surrounding fluid, it moves in
the same direction as if it only expelled fluid. This surprising phenomenon,
which we call "Machian propulsion," is explained by conservation of momentum:
the outflow efficiently transfers momentum away from the device and into the
surrounding medium, while the inflow can do so only by viscous diffusion.
However, many previous theoretical discussions have focused instead on the
difference in the shapes of the outflow and the inflow. Whereas the argument
based on conservation is straightforward and complete, the analysis of the
shapes of the flows is more subtle and requires conservation in the first
place. Our discussion covers three devices that have usually been treated
separately: the reverse sprinkler (also called the inverse, or Feynman
sprinkler), the putt-putt boat, and the aspirating cantilever. We then briefly
mention some applications of Machian propulsion, ranging from microengineering
to astrophysics.
