In 1817, when the place of Second Pupil was freed, Mossotti was advanced to
Second Pupil and started to receive a salary. In 1817 and 1818 Mossotti was
working at the ephemerides, and published some supplements to his Nuova
analisi del problema di determinare le orbite dei corpi celesti. His fame was
by now truly international: in 1817 Gauss published in Göttingen a
review
of the aforementioned paper, and in 1818 Encke
published in Tübingen an abridged German translation of the paper and the
supplements.
On 22 August 1818 (M17) de Cesaris wrote to the Government proposing the promotion of the Second
and Third Pupils to (respectively) First and Second, and the appointment of
Gabrio Piola as Third Pupil. It is clear from the letter that Mossotti was
kept in exceptionally high consideration by Brera Observatoire. About half of
the letter is devoted to the praise of Mossotti's fame and ability, in which
the director of the Observatory writes in fact a short eulogy of his pupil,
saying that he is a man of ``wise conduct and continuous application", that he
tackles theory as well as practical observations, and both to a very high
level indeed. There is a short account of his two main publications, the paper
Sopra il movimento di un fluido elastico che sorte da un vaso, which in
de Cesaris' opinion shows ``how much he [Mossotti] is advanced in the art of
sublime mathematics", and the appendices to the ephemerides and their
supplements (Nuova analisi del problema di determinare le orbite dei
corpi celesti), ``full of elegant formulae and profound calculations, and
whose abstracts were published in the German journals of Göttingen and of
the Baron of Lindenau". De Cesaris goes on pleading for a high salary to be
assigned to Mossotti. In the rest of the letter, de Cesaris devotes to the
advancement of the Third Pupil (Enrico Brambilla) to Second Pupil only a very
short paragraph, saying that Brambilla is very good in calculations and
necessary to the Observatoire because he speaks German fairly well. For the
occupation of the place of Third Pupil de Cesaris recommends Gabrio Piola, a
young Milanese nobleman who had previously studied in Pavia with Brunacci,
saying that Piola had been helping in the Observatoire for the previous two
years.
The reply of the Government dates 13 April 1819 and nominates Mossotti as First Pupil and Brambilla as Second Pupil. For the place of Third Pupil the Government organized an examination to be taken in the universities of Padua and Pavia. The recommendation of de Cesaris probably bore considerable importance as there were only two candidates to the examination, one of which was obviously Piola, who finally got the job (not to mention the fact that the examination marker was de Cesaris himself!).
With Mossotti's nomination to First Pupil a long quarrel between Brera and the
Government began over Mossotti's salary. In the nomination letter, Mossotti's salary had been fixed
to 1200 florins per annum, but when he went to collect his pay they only gave
him 1200 lire (one lira was worth about a third of a florin). Mossotti
informed de Cesaris who wrote to the Government, which replied that the copist
had made a mistake. After many pleas from Brera and negative replies from the
Government, there came a letter from the Government saying with threatening
tones that Mossotti's salary had been fixed at 1200 lire, i.e. 400 florins per
annum, and there could be no further appeal: that decreed the end of the
disagreement.
In 1819 Mossotti published another interesting paper as an appendix to the Effemeridi astronomiche di Milano
per l'anno 1820, in which, in order to explain the difference in the results
of the calculations made by Littrow and Piazzi to determine the diameter of
the sun, Mossotti advances the supposition that the sun is in fact an
ellipsoid and not a sphere. According to Mossotti, that is the ``simplest and
most probable cause" to explain the fact that people in different places
observe different diameters.