The reason for this happening is because of cohesion and adhesion. Meniscus is the curve of the upper surface in any liquid. Curving can be convex or concave. Cohesion is the ability of water molecules to cling to each other because of hydrogen bonds. Adhesion is the ability of water to cling to other polar surfaces, as a result of water's polarity.

So when water is placed into a glass container, surface tension attracts the water molecules to the sides of the glass and because adhesion overcomes the forces of cohesion the water climbs the glass slightly. This does not happen with Mercury because the opposite of water happens. The forces of cohesion are stronger than its attraction to the glass, it's adhesion. It have the opposite effect meaning it will be convex.