Explanation:
To measure the magnetic bearing of a line, a theodolite should be provided with a tabular or trough compass. The magnetic bearing refers to the angle between the magnetic north and a line measured from a point. The theodolite is a precision instrument used in surveying and engineering to measure angles in both the horizontal and vertical planes. It consists of a telescope mounted on a rotating base, with graduated circles for measuring angles.

Importance of a compass in measuring magnetic bearing:
A compass is an essential component of a theodolite for measuring magnetic bearings. The compass provides the orientation reference by indicating the magnetic north direction. This is important because magnetic north may deviate from true north due to variations in the Earth's magnetic field.

Types of compasses in theodolites:
There are different types of compasses used in theodolites, but for measuring magnetic bearings, a tabular or trough compass is required. These compasses consist of a circular disc or a trough filled with liquid (usually alcohol or oil), with a magnetic needle or card floating in it. The needle aligns itself with the magnetic field, indicating the magnetic north direction.

Procedure for measuring magnetic bearing:
To measure the magnetic bearing of a line using a theodolite with a tabular or trough compass, the following steps can be followed:

1. Set up the theodolite at a known point or reference station.
2. Level the theodolite using the built-in spirit level.
3. Sight the line or object for which the magnetic bearing is to be measured using the telescope.
4. Read the horizontal angle indicated by the graduated circles on the theodolite.
5. Note the direction indicated by the compass needle or card.
6. Add or subtract the compass reading from the horizontal angle, depending on the direction of the compass needle relative to the graduated circles.
7. The result is the magnetic bearing of the line or object.

Conclusion:
In order to measure the magnetic bearing of a line accurately, a theodolite should be equipped with a tabular or trough compass. This compass provides the necessary reference for determining the magnetic north and measuring angles relative to it. The theodolite, along with the compass, allows for precise measurements of magnetic bearings in surveying and engineering applications.