Put in your Note-Book:
The sextant has the following parts: (Instructor points to each.)
| 1. Mirror | 6. Handle |
| 2. Telescope | 7. Sliding Limb |
| 3. Horizon Glass | 8. Reading Glass |
| 4. Shade Glasses | 9. Tangent Screw |
| 5. Back Shade Glasses | 10. Arc |
In getting angles of land-marks or buoys, the sextant is held by the handle No. 6 in a horizontal position. The vernier arrow in the sliding limb is set on zero. Now, suppose you wish to get the angular distance between two lighthouses as seen from the bridge of your ship. (Draw diagram.)
Look at one lighthouse through the line of sight and true horizon part of the horizon glass. Now, move the sliding limb along the arc gradually until you see the other lighthouse in the reflected horizon of the horizon glass. When one lighthouse in the true horizon is directly on top of the other lighthouse in the reflected horizon, clamp the sliding limb. If any additional adjustment must be made, make it with the tangent screw No. 9.
Now look through the reading glass No. 8. You should see that the arc is divided into degrees and sixths of degrees in the following manner:
Now, as every degree is divided into sixty minutes, one-sixth of a degree is 10 minutes. In other words, each of the divisions of a degree on this arc represents 10 minutes.
Now on the vernier in the sliding limb, directly under the arc, is the same kind of a division. But these divisions on the vernier represent minutes and sixths of a minute, or 10 seconds.