  Euler considers the following problem: A boat with a perfect rudder moves at
constant speed across a stream flowing in straight fillets at assigned speeds.
Assuming that the downstream velocity of the boat equals that of the river, how
should the rudder be set so that the boat traverses a given path? He works out
various instances, one of which gives rise to a variational problem, in detail.
(From Clifford Truesdell's An idiot's fugitive essays on science: methods,
criticisms, training, circumstances.)
