or the _Ordo_, and that the office partly recited is not the office of
the current day, what is to be done? If the priest has without fault
made the mistake of reciting some office not ascribed to the current
day, he is not bound to repeat the part already recited (e.g.,
Matins); it is sufficient, valid and lawful to follow the correct
office in the following Hours. The priest reciting is not bound to
repeat even part of an hour, if he finds out his mistake during the
recitation of even a small hour. And he may finish the psalm or hymn or
prayer which he was reciting when he discovered his mistake, and he may
then take up the correct office at the part or hour at which he leaves
off, or he may finish the Hour at which he was engaged. The former
solution of the difficulty seems the better, as it more accurately
agrees with the maxim, _error corrigatur ubi apprehenditur_. If the
error in the selecting of the office has been wilful, say, through gross
carelessness, and is the fault of the priest who changes a notable part
of a canonical Hour, he is obliged--the more probable opinion
teaches--to repeat the full Hour, and this obligation binds under pain
of venial sin--_i.e._, the obligation to recite the office in the
prescribed manner.

What is a person bound to do who forgets part of an Hour--is he obliged
to repeat the full Hour?

He is bound to recite the part forgotten only, unless the mistake be
made through gross carelessness, and unless it be a considerable part
(e.g., two nocturns); in that case he is bound under pain of venial
sin to repeat the full Hour. If a person say the same Hour (e.g.,
Terce) twice, may he compensate for extra labour by the omission of an
equivalent part (e.g., None)? Such omission is unlawful; he must
recite all the Hours without omission (Scavini, 391).