To ensure more accurate reading of the barometer, a secondary scale or =vernier= is used, which slides upon the principal scale. This vernier is so graduated that 25 of its divisions correspond to 24 of the divisions on the fixed scale. The fixed scale is divided into inches, tenths (·1), and half-tenths (·05). Each division of the movable scale or vernier is therefore shorter than each division of the scale by 1∕25 of ·05, _i.e._ ·002 inch. Consequently the vernier shows differences of two thousands of an inch. [Illustration: FIG. 47. FORTIN BAROMETER. _a_—Attached thermometer. _b_—Screw of vernier. _c_—Screw for setting level of cistern.] _Method of reading Fortin’s Barometer._—First note the reading of the attached thermometer; next turn the screw at the bottom of the cistern, so that the ivory point just touches the surface of the mercury. Next adjust the vernier by means of the rack and pinion at the side of the barometer (Fig. 47) so as to bring its two lower edges exactly on a level with the convex surface of the mercury. In reading the barometer, first read off the division next below the lower edge of the vernier. In Fig. 48 this is 29·05. Then the true reading is 29·05 _plus_ the vernier indication. Next look along the vernier until one of its lines is found to agree with a line on the scale. In Fig. 48 this is at the fourth division on the vernier. But each of the figures marked on the vernier counts as 1∕100 (·01), and each intermediate division as 2∕1000 (·002); hence the reading of the vernier will be ·008 inch, and the reading of the barometer 29·05 + ·008 = 29·058 inch. If two lines on the vernier are in equally near agreement with two on the scale, the intermediate value should be adopted. Certain _corrections_ are required in the actual reading for (1) index error; (2) temperature; and (3) height above sea-level. The _index error_ is found by comparison with a recognised standard at Kew. Correction for _temperature_ is required. Every barometer has a thermometer attached, and the reading is reduced to the standard temperature of 32° F, by means of tables such as are given on page 32 of Marriott’s _Hints to Meteorological Observers_. The height of the _cistern_ of the barometer above _sea-level_ should always be exactly obtained. The correction necessary to reduce observations to sea-level (_i.e._ mean half-tide level at Liverpool), depends on the temperature and pressure of the air, as well as on the altitude. The data for this correction are given in Table III. of Marriott’s _Hints_. [Illustration: FIG. 48. SCALE OF BAROMETER (TO RIGHT) AND OF VERNIER (TO LEFT).]