Будь ласка 10 питань до текстуLevelling rods. Levelling rods are of two types, target and self – reading; the former is read by a rodmen, the latter by a levelmen. The commonest forms of target rods are the Boston, the New York and the Philadelphia; the Philadelphia can be used also as a self- reading rod. Most rods are made of two or more sections, so that they may be extended. The target of the Boston rod is permanently fastened to one strip of the rod. The other target rods are always used right – side up and have movable targets. The New York rod has a vernier for reading to a thousandth of a foot; most Philadelphia rods have a scale resembling a vernier, which gives the hundredths and makes it easy to estimate thousandths, thus serving every purpose of a vernier. All these rods are usually 6 or 7.5 ft. long and are extensible for readings up to 11 or 13 ft. They are also made 3 ft. long, with extension to 5 ft. and 5 ft. long with extension to 9 ft. for use underground. A self – reading rod has graduations (usually every tenth of a foot) painted on its face, so that it can be read through a telescope by the levelman. The figures on the face of the rod are usually of a definite height (0.06 or 0.08 ft. ) and of a thickness of 0.01 or 0.02 ft., thus aiding the levelman to estimate hundredths of a foot or even closer. Engineer*slevel is a telescope to which a delicate spirit level is attached parallel to the line of sight, so that when the bubble is in the centre, the line of sight is horizontal. The two common types are the Y- and dumpy – levels. In both the telescope is mounted on a vertical axis about which the telescope can swing horizontally and is leveled by 4 levelling screws. The Y- level has its spirit level attached to the telescope which rests in two Y- shaped supports fastened to a horizontal bar rigidly connected to a vertical axis. The telescope can be taken out of the Ys, turned end for end. The dumpy – level has its vertical axis, a horizontal bar and supports for the telescope all cast in one piece to which a spirit level is attached. The dumpy – level will stand much rougher use than Y – level.
1. Which main types of levelling rods do you know? 2. What are the commonest forms of target rods? 3. Can Philadelphia rod be used as a self-reading rod or not? 4. Target of which rod is permanently fastened to one strip of it? 5. What length does rod usually have for underground use? 6. Why does self–reading rods have graduations on its face? 7. Why does figures on the face of the rod have definite height and thickness? 8. What is the difference between Y and dumpy levels? 9. Where in Y-level is telescope rest? 10. What in dumpy level is spirit level attached to?
