The World's Smallest Mercury Monitor ....The HERMES has dimensions of 4 x 3 x 1.5 inches and weighs only 0.75 lb (340 g). It has a built in GPS so that mercury measurements may be logged continuously along with geographic location. By folding the optical path in the shape of a "U," it was possible to achieve a 15-centimeter path length, enabling extremely sensitive measurements of mercury in ambient air.