Haydn scholarship has long been aware that Joseph Haydn was not the first to set Marchimont Nedham's Invocation of Neptune text. The libretto of Friedrich Hartmann Graf's ode even survived in the British Library, but further settings by Johann Christian Fischer and Ignaz Pleyel remained in complete obscurity. An anonymous Invocation composition from Haydn's personal music collection (now in the National Széchényi Library, Budapest) inspires reconsideration of this problem. The work was probably performed on 23 May 1792 in the Hanover Square Rooms at 'Mr. Fischer's Night' and may well be Fischer's composition (if not one in fact co-written by Fischer and Pleyel, which could explain our complete lack of information about the performance of a third pre-Haydn Invocation setting). Apart from clarifying some details of the genesis of Haydn's Invocation, comparison with the anonymous setting also allows us to better appreciate the sensitive relationship between text and music in Haydn's work.
Pohl, Carl Ferdinand (1863) "Über Originalhandschriften von Mozart, Haydn etc." Allgemeine Musikalische Zeitung, Neue Folge 1, pp. 853-856.