What we need is more time - Dumbledore

'It is your turn to listen, and I beg you will not interrupt me, because there is very little time,' he said quietly. 'There is not a shred of proof to support Black's story, except your word - and the word of two thirteen-year-old wizards will not convince anybody. A street full of eye-witnesses swore they saw Sirius murder Pettigrew. I myself gave evidence to the Ministry that Sirius had been the Potters' Secret Keeper.'
'Professor Lupin can tell you -' Harry said, unable to stop himself.
'Professor Lupin is currently deep in the Forest, unable to tell anyone anything. By the time he is human again, it will be too late, Sirius will be worse than dead. I might add that werewolves are so mistrusted by most of our kind that his support will count for very little - and the fact that he and Sirius are old friends -'
'But -'
'Listen to me, Harry. It is too late, you understand me? You must see that Professor Snape's version of events is far more convincing than yours.'
'He hates Sirius,' Hermione said desperately. 'All because of some stupid trick Sirius played on him -'
'Sirius has not acted like an innocent man. The attack on the Fat Lady - entering Gryffindor Tower with a knife - without Pettigrew, alive or dead, we have no chance of over-turning Sirius' sentence.'
'But you believe us.'
'Yes, I do,' said Dumbledore quietly. 'But I have no power to make other men see the truth, or to overrule the Minister for Magic...'
Harry stared up into the grave face and felt as though the ground beneath him was falling sharply away. He had grown used to the idea that Dumbledore could solve anything. He had expected Dumbledore to pull some amazing solution out of the air. But no... their last hope was gone.
'What we need,' said Dumbledore slowly, and his light-blue eyes moved from Harry to Hermione, 'is more time.'
'But -' Hermione began. And then her eyes became very round. 'OH!'

HERMIONE'S SECRET - Pg 417