I
adjective
1
BAD : The girl was of middle height and was wearing a blue suit.
GOOD : The girl was of medium height and was wearing a blue suit.
◆◆◆
When you are talking about someone's height, weight, etc, use medium (NOT middle ): 'The man that police wish to question is of medium height and has a small black moustache.'
2
BAD : The wallet was middle brown and contained sixty dollars.
GOOD : The wallet was medium brown and contained sixty dollars.
◆◆◆
When you describe the colour of something, use medium (NOT middle ): 'a medium grey suit' (= neither light grey nor dark grey but somewhere in between).
II
noun
BAD : On the middle of September she is going on maternity leave.
GOOD : In the middle of September she is going on maternity leave.
◆◆◆
in the middle of (NOT on ): 'The cat dropped the mouse right in the middle of the kitchen floor.' 'What do we do if the fire alarm starts ringing in the middle of an examination?'