Bjerknes realized that the velocity field generated by an expanding particle in an incompressible fluid has the same geometrical structure as the electric field generated by a positively charged particle, and that the same applies for contracting particle and a negatively charged particle. 
In the case of an oscillating motion, Bjerknes argued that two particles that oscillate in phase generate a velocity field that is geometrically equivalent to the electric field generated by two particles with the same charge, whereas two particles that oscillate in an opposite phase will generate a velocity field that is geometrically equivalent to the electric field generated by particles with an opposite sign.
Bjerknes then writes:This result is counter to our intuition, as it demonstrates that bodies oscillating in phase exert an attractive force on each other, despite creating a field akin to that of identically charged particles. This result was described by Bjerknes as "Astonishing".