An exactly similar expression holds good in **hydrokinetics**, provided that for the electric potential we substitute velocity potential, and for the electric force the velocity of the liquid.

In the theory of surfaces, in **hydrokinetics**, heat-conduction, potentials, &c., we constantly meet with what is called " Laplace's operator," viz.

Neither of these men professed to employ the calculus itself, but they recognized fully the extraordinary clearness of insight which is gained even by merely translating the unwieldy Cartesian expressions met with in **hydrokinetics** and in electrodynamics into the pregnant language of quaternions.