....^9.1

We can reproduce Equation (7.194) by realizing that $\partial v_x/\partial x+ \partial v_z/\partial z = V^{\,-1}\,\partial V/\partial t$, where $V$ is the volume of a small co-moving volume element. Combining this expression with the definition of bulk modulus, $K=\rho\,\partial p/\partial\rho=-V\,\partial p/\partial V$, we obtain $\partial p/\partial t = -K\,(\partial v_x/\partial x+\partial v_z/\partial z)$.

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... gives ^9.2

Taking the finite compressibility of water into account, this equation generalizes to $\partial^{\,2}\phi/\partial t^{\,2} = c^{\,2}(\partial^{\,2}\phi/\partial x^{\,2}+\partial^{\,2}\phi/\partial z^{\,2})$, where $c=\sqrt{K/\rho}$ is the velocity of sound. However, the left-hand side of the general equation is negligible for gravity waves, whose propagation velocities are much less than $c$.

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...quanta,^11.1

Plural of quantum: Latin neuter of quantus: how much?

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