Publisher's Synopsis
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1913 Excerpt: ...scale of the section is generally much larger than the horizontal, in order to magnify the irregularities of the ground--for instance, a common proportion is 4 in. to the mile horizontal, and 100 ft. to the inch vertical. The Ordnance bench mark, as cut on piers and walls, is shown in Fig. 133. In reading to an Ordnance bench mark, as shown on the pier in Fig. 134, if the wire cuts above the Fig. 135.--Reading downwards from string course. centre of the horizontal groove, the blade of a pocket knife can be put in the groove and the staff rested upon it while the reading is taken; but if the wire cuts below the mark, the operation will be as shown in the diagram. The staff is rested on the ground against the pier and the reading taken as 0-82, then the centre of the groove is sighted across to read 1-53, then the difference between 0-82 and 1-53 = 0-71, entered in the book as-0-71, because it is a reading from above downwards. Another case of reading downwards occurs when the underside of a string-course is made a bench mark, as in Fig. 135, giving a minus reading of 8 37. In reducing levels a minus reading is treated in the opposite manner to an ordinary reading, deduct instead of add, or vice versa. The preceding levels (Fig. 136) taken at Primrose Hill, London, show examples of main and cross sections, and the method of plotting them is shown in the accompanying section (Fig. 137). QUESTIONS ON CHAPTER XIX. 1. Explain what is meant by running or flying levels. 2. Define the terms "check levels," "bench mark," "Ordnance datum." 3. What is the level of Trinity High Water Mark in London compared with Ordnance datum i 4. Show by a sketch the columns used in a level book, and give three lines of entries. 5. When are "intermed...
