# Why would prefer to use the mercury instead the water in a sphygmomanometer?

Because $1 \cdot a t m$ supports a column of mercury $760$ $m m$ high..........
But given the density of mercury compared to water (${\rho}_{\text{Hg"=13.55*g*cm^-3; rho_"water}} = 1 \cdot g \cdot c {m}^{-} 3$, it would require a column of water approx. $10 \cdot m$ high to support a pressure of $1 \cdot a t m$.
Now sphygmomanometers take blood pressure measurements; typically we measure $50 - 60 \cdot m m$ above or below atmospheric pressure. A measurement of a water column would still be prohibitively high.