These new types of leveling instruments use a free mercury surface as a mirror to assure that the line of sight through {HEAD ~ the center} of {TAIL ~ the cross hairs} is horizontal, or has a very small but constant inclination from the true horizontal for all pointings to the leveling rods 
These new types of leveling instruments use a free mercury surface as a mirror to assure that the line of sight through { HEAD ~ the center } of { TAIL ~ the cross hairs } is horizontal , or has a very small but constant inclination from the true horizontal for all pointings to the leveling rods
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH