\begin{eqnarray}
\left(\nabla^2\frac{1}{\chi}\frac{\partial}{\partial t} \right) T(\mathbf{r}, t)&=&\frac{1}{\kappa}H(\mathbf{r}, t),\\\\
\left(\nabla^2\frac{1}{c^2}\frac{\partial^2}{\partial t^2}\right)p\left( \mathbf{r}, t \right)&=&\rho\beta\frac{\partial^2}{\partial t^2}T\left(\mathbf{r}, t\right).
\end{eqnarray}
These are the Phenomena of Springs and springy bodies, which as they have not hitherto been by any that I know
reduced to Rules, so have all the attempts for the explications of the reason of their power, and of springiness
in general, been very insufficient.
Robert Hooke, De Potentia Restitutiva (1678)
