  The classical Cepheid $l$ Carinae is an essential calibrator of the Cepheid
Leavitt Law as a rare long-period Galactic Cepheid. Understanding the
properties of this star will also constrain the physics and evolution of
massive ($M \ge 8$ $M_\odot$) Cepheids. The challenge, however, is precisely
measuring the star's pulsation period and its rate of period change. The former
is important for calibrating the Leavitt Law and the latter for stellar
evolution modeling. In this work, we combine previous time-series observations
spanning more than a century with new observations to remeasure the pulsation
period and compute the rate of period change. We compare our new rate of period
change with stellar evolution models to measure the properties of $l$ Car, but
find models and observations are, at best, marginally consistent. The results
imply that $l$ Car does not have significantly enhanced mass-loss rates like
that measured for $\delta$ Cephei. We find that the mass of $l$ Car is about 8
- 10 $M_\odot$. We present Hubble Space Telescope COS observations that also
differ from measurements for $\delta$ Cep, and $\beta$ Dor. These measurements
further add to the challenge of understanding the physics of Cepheids, but do
hint at the possible relation between enhanced mass loss and ultraviolet
emission, perhaps both due to the strength of shocks propagating in the
atmospheres of Cepheids.
