Barometer
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An instrument which records the weight or pressure of the atmosphere. The mercurial barometer consists of a glass tube 34 inches long, closed at the top, and exhausted of air. The lower end of the tube is immersed in a cup of mercury (quicksilver), which the pressure of the atmosphere causes to rise in the tube, on one side of which, on the frame, a graduated scale is found, embracing the range of oscillations. (See Aneroid Barometer.)
THE QUADRANT OF REFLECTION
1. Sliding Limb.
2. Mirror.
3. Horizon Glass.
4. Sight Vane.
5. Shade Glasses.
6. Adjusting Screw.
7. Adjusting Screw on Mirror.
8. Index Tangent Screw.
9. Arc.
0. Frame.
The vernier is set on the sliding limb against the arc of the instrument
