I have a minor question concerning Greek. What does the ~-sign that is sometimes positioned over a letter (e. g.: ã) indicate?
Thank you. 
Did you mean a circumflex or tilde? If so,then it is probably the περισπομένη, which (in Attic Greek) symbolised the rising then falling of pitch in a long syllable. It is only used on a long vowel.
If you meant a macron, then it symbolises a long alpha, iota or upsilon.
I’m afraid I don’t quite follow you. Does this ~ equal a circumflex, and does it therefore function as an accent? Or is this ~ called a macron? And, if a macron makes an alpha, iota or upsilon long, how would that compare with the circumflex, as it does the same thing? I hope not to burden you with these tedious questions.
I was told that ~ is another way to write the circumflex, as opposed to ^
Is that so? That might very well be the case. If the ~ would indeed be a separate, relevant, Greek character, I should expect to be able to find it in Smyth’s grammar, but I cannot. This sheds a favourable light on your point of view, benissimus.
I agree with benissimus. The ~ occurs in some weird greek fonts for the circumflex.
Macrons do not mark tone, while circumflexes do. Acute and grave accents may also be found on long syllables.
~ is the only symbol in all my books for the circumflex, they don’t seem to use ^as a sign for the circumflex here at all…