  We report the results of a combined study of the normal state resistivity and
superconducting transition temperature $T_c$ of the unconventional
superconductor Sr$_2$RuO$_4$ under uniaxial pressure. There is strong evidence
that as well as driving $T_c$ through a maximum at $\sim$3.5 K, compressive
strains $\varepsilon$ of nearly 1 % along the crystallographic [100] axis drive
the $\gamma$ Fermi surface sheet through a Van Hove singularity, changing the
temperature dependence of the resistivity from $T^2$ above and below the
transition region to $T^{1.5}$ within it. This occurs in extremely pure single
crystals in which the impurity contribution to the resistivity is $<$100
n$\Omega$cm, so our study also highlights the potential of uniaxial pressure as
a more general probe of this class of physics in clean systems.
