According to the Time Traveller in H.G. Wells' The Time Machine, what is the Fourth Dimension?

In Chapter 1 of The Time Machine the
reader is introduced to Wells' definition of the Fourth Dimension. In Wells' story, the
Time Traveller introduces the idea of time as the Fourth Dimension (current science now
holds that time, in space-time, provides a fourth dimension to the universe). The Time
Traveller's audience, the Medical Man, the Psychologist, the Very Young Man, the
Provincial Mayor, Fillby and Hillyer all protest that time cannot be a dimension because
it is impossible to willfully move through it in the way that altitude, latitude and
longitude can be intentionally moved through. The Time Traveller quiets their
protestations by having the Psychologist pull a lever on a small box. When the
Psychologist does so, the box vanishes and the group is told the the box has left them
and is traveling through space, an announcement followed by his declaration to similarly
travel through the Fourth Dimension of time by using a bigger version of the small time
traveling box.