The Time Traveller smiled.  ’Are you sure we can move freely in Space?  Right and left we can go, backward and forward freely enough, and men always have done so.  I admit we move freely in two dimensions.  But how about up and down?  Gravitation limits us there.’

‘Not exactly,’ said the Medical Man.  ‘There are balloons.’

’But before the balloons, save for spasmodic jumping and the inequalities of the surface, man had no freedom of vertical movement.’

‘Still they could move a little up and down,’ said the Medical Man.

‘Easier, far easier down than up.’

’And you cannot move at all in Time, you cannot get away from the present moment.’

’My dear sir, that is just where you are wrong.  That is just where the whole world has gone wrong.  We are always getting away from the present moment.  Our mental existences, which are immaterial and have no dimensions, are passing along the Time-Dimension with a uniform velocity from the cradle to the grave.  Just as we should travel down if we began our existence fifty miles above the earth’s surface.’

‘But the great difficulty is this,’ interrupted the Psychologist.  ’You can move about in all directions of Space, but you cannot move about in Time.’

’That is the germ of my great discovery.  But you are wrong to say that we cannot move about in Time.  For instance, if I am recalling an incident very vividly I go back to the instant of its occurrence:  I become absent-minded, as you say.  I jump back for a moment.  Of course we have no means of staying back for any length of Time, any more than a savage or an animal has of staying six feet above the ground.  But a civilized man is better off than the savage in this respect.  He can go up against gravitation in a balloon, and why should he not hope that ultimately he may be able to stop or accelerate his drift along the Time-Dimension, or even turn about and travel the other way?’

‘Oh, this,’ began Filby, ‘is all—­’

‘Why not?’ said the Time Traveller.

‘It’s against reason,’ said Filby.

‘What reason?’ said the Time Traveller.

‘You can show black is white by argument,’ said Filby, ’but you will never convince me.’

‘Possibly not,’ said the Time Traveller.  ’But now you begin to see the object of my investigations into the geometry of Four Dimensions.  Long ago I had a vague inkling of a machine—­’

‘To travel through Time!’ exclaimed the Very Young Man.

’That shall travel indifferently in any direction of Space and Time, as the driver determines.’

Filby contented himself with laughter.

‘But I have experimental verification,’ said the Time Traveller.

‘It would be remarkably convenient for the historian,’ the Psychologist suggested.  ’One might travel back and verify the accepted account of the Battle of Hastings, for instance!’