In the less elaborate models, {HEAD ~ the mercury surface} is used to guarantee that the line of sight through the center of cross hairs has {TAIL ~ only a very small but constant inclination} with the true horizontal 
In the less elaborate models , { HEAD ~ the mercury surface } is used to guarantee that the line of sight through the center of cross hairs has { TAIL ~ only a very small but constant inclination } with the true horizontal
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH