To a certain extent it is a matter of careful pronunciation, but this is not always a safe or even a possible guide.

The vowels preceding or following the one on which the primary accent falls, sometimes called obscure vowels, are so slurringly pronounced that even a pedantic precision will hardly make it possible to indicate clearly which vowel is used.

The writer remembers seeing an examination paper written by a fourth year medical student in which the word _fever_ was spelled _fevor_.

A moment's thought will show that so far as pronunciation is concerned the word might be spelled _fevar_, _fevir_, _fevor_, _fever_, or _fevur_ without any appreciable difference.

The correct spelling is merely a matter of observation.
The author has on his desk at the moment of writing these lines half a dozen good books, each containing a set of rules for spelling.