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The equation of mass conservation is,

(16) 
By use of a vector identity, Eqn. 16 may be expressed as,

(17) 
This simply states that, at any point, the density increases if there is material flow into the region (∂/∂t > 0 for ∇.ρv < 0) and decreases with material flow away from the region (i.e., positive divergence).
James McAteer
20040114