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