  The time derivative of the circulation of a vector field $\boldsymbol{A}$
over a moving and deforming closed curve, $\frac {\mathrm{d}}{\mathrm{d}
t}\oint \boldsymbol{A} \cdot \mathrm{d} \boldsymbol{r}$, is computed in two
ways, with and without bringing the time derivative under the integral sign. As
a by-product, the computations reveal that the conceptualization of Faraday's
law of electromagnetic induction may depend on which of the two methods is
employed. The discussion presented provides an unexpected argument in favor of
Maxwell's mysterious choice for his electromotive intensity $\mathfrak E$, made
in Article 598 of his Treatise.
