Normal gravity কী?

Example 11.12 (A station in the hills)

At a station at latitude $45{}^{\circ}$ and $1200\,\mathrm{m}$ above sea level, the gravimeter gives $9.802\,597\,\mathrm{m}\,\mathrm{s}^{-2}$. Normal gravity there is $9.806\,200\,\mathrm{m}\,\mathrm{s}^{-2}$; the measured value is lower by $360.3\,\mathrm{m}\mathrm{Gal}$. The free-air correction is $0.3086\times1200 = 370.3\,\mathrm{m}\mathrm{Gal}$, so $\Delta g_{\mathrm{FA}} = -360.3 + 370.3 = +10.0\,\mathrm{m}\mathrm{Gal}$. The Bouguer correction is $0.112\times1200 = 134.4\,\mathrm{m}\mathrm{Gal}$, so $\Delta g_{\mathrm B} = -124.4\,\mathrm{m}\mathrm{Gal}$: there is less mass below this station than below a station at sea level, once the rock above sea level is removed.