There is usually a leather thong at each end of a tape, by which the tape can be held when the full length is being used. When only part of the tape is used, the zero end can be held by the thong, and the tape can be held at an intermediate point by means of a tape clamp handle, like those shown in figure 11-43.

When a tape is not supported throughout that is, when it is held aboveground between a couple of crew membersa correction must be applied for the amount of sag in the tape. To make this correction, you apply a certain amount of tension. Figure 11-44 shows two devices for applying a given amount of tension.

Figure 11-44.-Tension scale and spring balance.

The tension scale is graduated in pounds from 0 to 30. It is clipped to the eye at the end of the tape, and the tension is applied until the desired reading appears on the scale. A pair of staffs can be used to make the work easier. The rawhide thongs are wrapped around the staff at a convenient height and gripped firmly. The bottom end of the staff is braced against the foot (fig. 11-45) and the upper end tucked under the arm. Tension is applied by using the shoulder and leaning against the poles. The spring balance is used in a similar fashion for work of higher precision.

The stool device in figure 11-45 is called a tapping stool or chaining buck and is used in high-precision work. It is a metal three-legged stand with an adjustable sliding head and a hand wheel operated device for locking the plate (the top surface of the sliding head) in any desired position. A line is scribed on the plate. During taping operations, the head is moved until the scribed line is directly under a particular graduation on the tape; the handwheel is then used to lock the head. When the tape is shifted ahead to measure the next interval, the graduation is held exactly over the line until the next stool is adjusted and locked. The basic purpose of taping stools is to furnish stable, elevated surfaces on which taped distances can be marked accurately. When stools are not available, 2 by 4s or 4 by 4s are often driven into the ground for use as chaining bucks.

The length of a tape varies with the temperature, and the precision of a survey may require the application of corrections for this. For work of ordinary precision, you can assume that the

temperature of the tape is about the same as that of the air. For work requiring higher precision, a tape thermometer, like the one shown in figure 11-46, is attached to the tape. For very precise work, two thermometers, one positioned at each end, may be used. If the two indicate different temperatures, the mean between them is calculated and used.