I am a 25 year member of AGU. This is the first letter that I have found it necessary omitted. We are told, for example, that Bjerknes “expanded the circulation theorem to its now classic formulation, including terms for forces arising from the earth's rotation and, in approximate form, from friction.” (p.39). But we are not given more. It is a pity to omit this, and I suspect the treatment reflects constraints on length and mathematical notation imposed upon the author. This is at least hinted at in a footnote.