Page:Elementary Text-book of Physics (Anthony, 1897).djvu/20

6 from 0 on the divided circle to the whole number into which it is divided.

The cathetometer is used for measuring differences of level. A graduated scale is cut on an upright bar, which can turn about a vertical axis. Over this bar slide two accurately fitting pieces, one of which can be clamped to the bar at any point, and serve as the fixed bearing of a micrometer screw. The screw runs in a nut in the second piece, which has a vernier attached, and carries a horizontal telescope furnished with cross-hairs. The telescope having been made accurately horizontal by means of a delicate level, the cross-hairs are made to cover one of the two points, the difference of level between which is sought, and the reading upon the scale is taken; the fixed piece is then undamped, and the telescope raised or lowered until the second point is covered by the cross-hairs, and the scale reading is again taken. The difference of scale reading is the difference of level sought.

The dividing engine may be used for dividing scales or for comparing lengths. In its usual form it consists essentially of a long micrometer screw, carrying a table, which slides, with a motion accurately parallel with itself, along fixed guides, resting on a firm support. To this table is fixed an apparatus for making successive cuts upon the object to be graduated.

The object to be graduated is fastened to the fixed support. The table is carried along through any required distance determined by the motion of the screw, and the cuts can be thus made at the proper intervals.