  Let $u(x,t)$ be a (possibly weak) solution of the Navier - Stokes equations
on all of ${\mathbb R}^3$, or on the torus ${\mathbb R}^3/ {\mathbb Z}^3$. The
{\it energy spectrum} of $u(\cdot,t)$ is the spherical integral \[
  E(\kappa,t) = \int_{|k| = \kappa} |\hat{u}(k,t)|^2 dS(k), \qquad
  0 \leq \kappa < \infty, \] or alternatively, a suitable approximate sum. An
argument involking scale invariance and dimensional analysis given by
Kolmogorov (1941) and Obukhov (1941) predicts that large Reynolds number
solutions of the Navier - Stokes equations in three dimensions should obey \[
  E(\kappa, t) \sim C_0\varepsilon^{2/3}\kappa^{-5/3} \] over an inertial range
$\kappa_1 \leq \kappa \leq \kappa_2$, at least in an average sense. We give a
global estimate on weak solutions in the norm
  $\|{\mathcal F}\partial_x u(\cdot, t)\|_\infty$ which gives bounds on a
solution's ability to satisfy the Kolmogorov law. A subsequent result is for
rigorous upper and lower bounds on the inertial range, and an upper bound on
the time of validity of the Kolmogorov spectral regime.
