It is enough for the observer at a fixed station, and to such alone can the use of a Fortin barometer be recommended, to read the temperature on the thermometer attached to the barometer and to read the height of the mercury in the barometer tube. These two figures he is to enter in his note-book, and unless he is himself discussing the results, he should apply no correction whatever to them. The rules for observing, then, are:—
1. Read the attached thermometer and note the reading.
2. Bring the surface of the mercury in the cistern into contact with the ivory point which forms the extremity of the measuring rod by turning the screw at the bottom of the cistern. The ivory point and its reflected image in the mercury should appear just to touch each other and form a double cone.
3. Adjust the vernier scale so that its two lower edges shall form a tangent to the convex surface of the mercury. The front and back edges of the vernier, the top of the mercury, and the eye of the observer are then in the same straight line.
4. Take the reading, and enter the observation as read without either correcting it to freezing point or reducing it to the sea-level.
The scale fixed to the barometer is divided into inches, tenths, and half-tenths, so that each division on this scale is equal to 0.050 inch.
The small movable scale or vernier attached to the instrument enables the observer to take more accurate readings; it is moved by a rack and pinion. Twenty-four spaces on the fixed scale correspond to twenty-five spaces on the vernier; hence each space on the fixed scale is larger than a space on the vernier by the twenty-fifth part of 0.050 inch, which is 0.002. Every long line on the vernier (marked 1, 2, 3, 4, and 5) thus corresponds to 0.010 inch. If the lower edge of the vernier coincides with a line on the fixed scale, and the upper edge with the twenty-fourth division of the latter higher up, the reading is at once supplied by the fixed scale as in A (Fig. 5), where it is 29.500 inches. If this coincidence does not take place, then read off the division on the fixed scale, above which the lower edge of the vernier stands. In B (Fig. 5) this is 29.750 inches. Next look along the vernier until one of its lines is found to coincide with a line on the fixed scale. In B this will be found to be the case with the second line above the figure “2.” The reading of the barometer is therefore:—
| On fixed scale | 29.750 |
| On vernier (12 × .002) | .024 |
| Correct reading | 29.774 |
Should two lines on the vernier be in equally near agreement with two on the fixed scale, then the intermediate value should be adopted.