THE TRANSIT The engineer's transit consists of an alidade, carrying a line of sight, attached to an inner vertical circle (upper motion) which may be turned in an outer circle (lower motion). The lower motion carries a horizontal circle which is graduated to degrees that may be read to minutes by means of a vernier. The alidade includes the telescope and a magnetic needle with its graduated circle; it may be revolved while the graduated limb remains stationary. The instrument is supported by a tripod head by means of which it may be attached to a tripod. The complete transit should have a vertical circle graduated to degrees and vernier to minutes, and a level bubble attached to the telescope.
The transit is used for measuring horizontal and vertical angles. The attached bubble enables it to be used also as a level.
The telescope carries besides the regular cross hairs, equi distant from the horizontal one, two other horizontal cross hairs, known as stadia hairs. The object of the stadia cross hairs is the measuring of distances by reading an intercept on a graduated rod. In most instruments the stadia wires are so adjusted that an interval of one foot on the rod between the .cross hairs will indicate that the point where the rod is held is zoo feet from the objective lens of the instrument, to this the instrument constant is to be added.
Horizontal as well as vertical distances may be determined by means of stadia wherever a great degree of accuracy is not required. The transit is set up over a station and the inter cept on the rod is noted, as well as the angle shown by the vertical circle. To determine the horizontal and vertical distances, trigonometric formulx are used, as we have a right-angled triangle with an acute angle and the hypothenuse given. The calculations may be made by means of tables, diagrams, specially constructed stadia slide-rules, or Beaman Stadia Arc. As dia grams and slide rules cannot well be taken into the field, the use of the Beaman Stadia Arc or tables are recommended. (Fig. 24.) (Table XIV.) If the readings are, for vertical circle 20' and the inter cept on the rod is 2.58 feet, the stadia interval of the instrument being ioo and the instrument constant i.00 the method of cal culation is as follows: Hor. Dis. Ver. Dis.
From the table under 20' we get 93.01 25.50 Instrument constant Loo .27 the horizontal distance equals: 93.01 X 2.58 +.96 = 240.93 the vertical distance equals: 25.50X2.58-}-.27= 66.o6 .
This method of figuring is all that is necessary if the middle hair is placed upon the rod at a height equal to the height of the instrument above the ground. If this is not the case a cor responding correction must be made, which will be the difference between the height of instrument above the ground and the reading of the middle cross-hair, and this difference is to be added if the reading on the rod is less than the height of the instru ment, and subtracted if it is greater.
Adjustment of the Transit. Set up the transit, level up plate bubbles and turn the instrument If the bubbles stay in the center they are in adjustment, if not, correct by bringing bubbles towards the center one-half the distance, with adjusting pins, the other half by leveling screws.
Line of collimation. For this adjustment select a point on the ground to which the telescope is to be directed, clamp motion and plunge the telescope and place a mark on a well-defined object where the vertical cross-hair cuts a horizontally drawn line. With telescope still inverted, loosen motion and turn back and sight first point with telescope still inverted, then with motions clamped, plunge telescope again and make another mark on the same horizontal line as in the first case. If adjust ment is 0. K. the cross-hair will bisect the first point, if not, adjustment is to be made for one-quarter of the error by loosening the screws on the side of the telescope.
Horizontal axis. After instrument is set up, sight the instru ment on a plumb line or at a point up on the side of a building, depress telescope and set a point where the vertical line cuts a horizontally marked line along the side of the building. Revers ing the instrument in azimuth and altitude, sight again at first high point and depress instrument again, making another point along the horizontally drawn line on the side of the building. If both marks coincide the axis is in adjustment, if not, correct for one-quarter of the error by means of the horizontal reticule adjusting screws, moving the reticule in a direction that would apparently increase the error.
Attached level. This adjustment is the same as the one required in the adjustment of the level for the wye adjustment, but as the telescope and level in a transit cannot be turned end for end in the wyes, it is adjusted by the two-peg method. (This method is also used in similar adjustment for the Dumpy level.) The procedure is as follows: set up the instrument, level tele scope, set a peg 30o feet from the instrument and take the rod reading, set another point 30o feet from the instrument in the opposite direction and take the rod reading. The difference of elevation of . these two points may be determined from these figures even with the instrument out of 'adjustment, as the back and foresights are equal in length. Move instrument and set up one or two feet from one of these pegs, hold rod on peg, turn the telescope and looking into the object end, the reading on the rod in the small field of view may be noted by bringing the point of a pencil to the center. The target is set at this point, plus or minus the difference in the elevation of the two pegs, then set rod on the distant peg and if instrument is in adjustment, the horizontal cross-hair will cut the target exactly; if not, adjust for the whole error by means of the reticule adjusting screws.