27) What is the process of levelling while the difference of level between two points is determined by setting the levelling instrument mid way the point?

28) What is the term of sight, if the last sight taken on a levelling staff held over a point of unknown elevation before shifting the instruments?

29) What is the term of sight, if the sight taken between the back sight and fore sight on a levelling staff held over a point of unknown elevations?

30) What is the level that combine good features both the dumpy level and 'y' level?

31) What is the fundamental line in levels?