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It is essential to remove the plugs during the measurements, otherwise the communicating vessels principle is not applicable anymore and measurements will be wrong.For the measurement of differences in elevation between two points in the field, the tube water level is adapted.Air bubbles can be removed by tapping the tube with the finger. 42 The "communicating vessel" principle To set out a contour line with a Cube water level, the following procedure is used: The two staffs are placed back to back at the starting point marked with peg (A).Wherever the two staffs are set, the free water surfaces in the tube ends have the same level (see Fig. This is called the "communicating vessel" principle. After the air bubbles have been removed and the water has come to a rest, a mark is made on both staffs, indicating the water level (see Fig 43a). 43b Setting out a contour line, Step 2 The point where the staff is then standing is at the same level as the starting point. The procedure is repeated, starting from peg (B), to find the third point (peg C) of the contour line.EXAMPLE: The hand level consists of a 10-12 cm long tube with an eye piece at one end and two hair lines (one horizontal and the other vertical) at the other end. 47 A hand level When the operator looks through the eye-piece, the mirror inside the tube, reflects (on the right hand side) the position of the bubble of the carpenter level. 48 Use of the hand level For greater stability the instrument can be supported by a forked bush pole, with a metal plate attached to the bottom. 49a Setting out a contour line, Step 1 Looking through the hand level, the elevation of the horizontal hairline is marked on the ranging pole.Attached to the tube is a small carpenter level (see Fig. The instrument is made in such a way that when the bubble is in sight on the horizontal hair line, the instrument is horizontal and the line of sight is horizontal (see Fig. This assures that the instrument is always at the same height above the ground surface. The hand level is placed on the crotch of the forked pole and tilted slowly until the bubble is seen at the horizontal hair line (see Fig. This can be done by tying a piece of coloured cloth around the pole, or with some coloured tape. 49c Setting out a contour line, Step 3 The ranging pole is placed about 10 to 15 metres away from the instrument in the general direction of the contour line.This choice should be based on the required accuracy (a little difference in height means it is more accurate), the general slope of the area and the regularity of the general slope of the area.
Again, readings are made and entered in the book (see Fig. The last readings are made and written down in the book.
The spot thus found is 2 cm lower than the starting point and is marked with a new peg (peg B)(see Fig. The procedure is repeated until the end of the field is reached.
The succession of pegs thus placed form a line with a slope of 1% (see Fig. This line would be, after correction, the centre line of a ditch with a slope of 1%. 40b Setting out a slope, Step 2 The flexible tube water level, used for contour lines and measuring differences in elevation, consists of two staffs with a length of about 2 m and a transparent flexible tube of about 14 m long.
To be able to set out horizontal lines or lines with a constant slope, the elevation (or height) of two points on the line (preferably the starting and end points) must be known.
Suppose a horizontal line has to be set out between the Bench Marks A and B. The procedure is: Place boning rods on top of the two Bench Marks and on top of peg C. 35f Setting out a horizontal line, Step 6 The use of boning rods when setting out a slope is the same as described in 18.104.22.168 only, in this case, the Bench Marks A and B do not have the same elevation. When the difference in elevation and the horizontal distance between A and B are known, the slope can be calculated (see Volume I, Chapter 3 and Volume 2 Chapter 3 and sections 6.3 and 6.4).
Each staff is graduated in centimetres and used as a measuring staff. Suppose the difference in elevation between two points A and B has to be measured; A and B are less than 10 m apart.