{HEAD ~ These new types} of {TAIL ~ leveling instruments} use 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 
{ HEAD ~ These new types } of { TAIL ~ leveling instruments } use 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
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