Another SI note. It is based on the last issue of TPT (September 1997).
On page 324 E. Legendijk observes that the speed of light in the vacuo
is no longer measurable; its value has been declared to be 299792458 m/s.
The unit of length, meter, is defined to make this happen.
I may add that the same is true for the magnetic permeability of empty
space. We can measure epsilon_zero (for example, from the force with
which parallel plates of a charged capacitor attract in a vacuum)
but we can not measure mu_zero (for example, by using the Biot Savart
formula). The unit of current is defined to make mu_zero equal to its
declared value, as David Bowman reminded us. In the old days the relation
c^2=epsilon_zero*mu_zero was considered to be an experimental fact.