# Jos Stam's stable fluids — why is the timestep multiplied by the number of grid cells?

One more question about Jos Stam's GDC tutorial on stable fluids: in the advection step on page 8, the timestep for each dimension is implemented as dt * N, where N is the height or width of the grid. This corresponds to a position update:

$$x_{prev} = x - \frac{\Delta t}{h} \cdot v_x$$

where $$h = 1/N$$.

But why is $$h$$ involved in the backtracing of velocity? I thought the time evolution would just be given by $$x = x_{prev} + \Delta t \cdot v_x$$ without involving the grid size.