$3$ Fig. $3\mathrm{.}1$ shows a mercury barometer correctly set up to measure the atmospheric pressure in mmHg $($millimetres of mercury$).$

riy. $0\mathrm{.}1$

$($a$)($i$)$ State what is in the space labelled P $.$

$($ii$)$ On Fig. $3\mathrm{.}1,$ mark carefully the length which gives the atmospheric pressure.

$($iii$)$ The tube containing mercury is pushed further down into the dish.

State what happens, if anything, to the vertical distance between the mercury surface in the tube and the mercury surface in the dish.