The mercury barometer is an instrument used to measure atmospheric pressure through the rising or falling level of mercury within a glass tube.
Constructing one is no tricky ordeal: all you need is a meter tall class tube inverted over a trough filled with mercury.
Once this whole set-up is placed out in the open, the atmosphere applies pressure on the mercury surface in the trough. This will change the amount of liquid pushed inside the glass tube, until an equilibrium point is reached wherein the mercury column becomes stabilized - that is when the atmospheric pressure exerted outside on the trough is equal to the pressure exerted inside the glass tube due to the liquid column:
Mercury is a good choice for us because it has high density, meaning the height of the entire column of liquid can be a reasonable size (not too large, not too small!) It is also interesting to note that because the mercury barometer relies solely on the height of mercury inside the tube, we find it more convenient to measure the atmospheric pressure in terms of height of mercury Hg (chemical symbol). In that regard, atmospheric pressure at sea level is 76 mm Hg; at higher altitudes (with low atmospheric pressure), the level decreases; at higher atmospheric pressures, the level rises.