Fig. 254.—Ruling of omnimeter field-book.
Larger image
582.—Field-book.—The field-book as shown above, Fig. 254, was recommended by the inventor.
583.—Mode of Operating with the Omnimeter.—Carefully set the instrument up at its station in perfect adjustment as a theodolite, noting the departure point upon the scale reading through the microscope. Place the stadium in a vertical position at the point to which measurements are required. Direct the telescope so that the horizontal web cuts the upper line of the stadium, and lightly clamp it. Now read the microscope and record the reading as observed in the field-book. Unclamp the telescope and take the reading of the lower point of the staff and record this. Record the bearing of the instrument on the horizontal circle as with a theodolite.
584.—To Determine the Horizontal Distance in Feet.—Divide the constant radius of 1,500,000 given before by the difference of the two readings of the stadium mark, which are 10 feet apart. For example:—
| First reading | of scale | 67,500, | micrometer | 235 = | 67,735 |
| Second | " | 64,000, | " | 450 = | 64,450 |
| Difference | 3,285; |
then
1,500,000 3285 = 456·6 feet distance.
The process is somewhat simplified by logarithms, as we have only the log. of the difference to subtract from the constant, the 1,500,000 mantissa of which is 1,760,913. Thus—
| log. 1,500,000 | 6·1760913 | |
| log. 3,285,000 | 3·5165354 | |
| 2·6595559 | = 456·6 feet. |