river-spanning museum

according to the relevant page of Jacques Bertrand’s ‘Page francophone des expositions universelles et internationales’, part of the River Seine was used by the Exposition du Centenaire in Paris in 1889, so it seems that James is here referring to that structure, whatever it was, between the Champs de Mars, at the foot of the newly constructed Eiffel tower, and the Trocadero onthe opposite bank of the river