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Published online by Cambridge University Press: 14 August 2015
The slow decline of surface brightness along many large-scale jets indicates that their internal pressures are determined largely by dissipation. If dissipation arises from a viscous interaction between a jet and its environment, then the observed degree of collimation enables one to constrain the nature of the viscous stress. Simple phenomenological models of the stress account for the frequently observed “gaps” and provide a means of slowing down jets without their becoming decollimated.