| Height in Feet. | Barometer in Inches. | Height in Feet. | Barometer in Inches. |
| 0 | 31 | 6000 | 24·875 |
| 250 | 30·717 | 7000 | 23·979 |
| 500 | 30·436 | 8000 | 23·125 |
| 750 | 30·159 | 9000 | 22·282 |
| 1000 | 29·883 | 10,000 | 21·479 |
| 1500 | 29·340 | 11,000 | 20·706 |
| 2000 | 28·807 | 12,000 | 19·959 |
| 2500 | 28·283 | 13,000 | 19·236 |
| 3000 | 27·769 | 14,000 | 18·535 |
| 4000 | 26·769 | 15,000 | 17·853 |
| 5000 | 25·804 |
It may be generally observed that the more open the scale the less altitude can be obtained by a single revolution of the hand; therefore the more points can be taken per 1000 feet. Thus, with an altitude barometer reading to 3000 feet, readings can be pointed in construction at every 250 feet; with one of 6000 feet, at every 500 feet; and over this at every 1000 feet.
834.—Movable Altitude Scale.—In this the altitude scale revolves so as to be able to set it at zero for ascending from any point. As the barometrical scale diminishes, it is necessarily inaccurate, and cannot therefore be used upon a surveying aneroid; but the plan is pleasant for approximate measurements for amusement in making ascents. It is only mentioned here for the reason that the inaccuracy of the movable scale is not always recognised.
835.—Adjustment of the Aneroid.—There is a screw at the back of every aneroid somewhere under the point A, [Fig. 394], by means of which an aneroid may be brought to the reading of a mercurial barometer at the position the mercury may be read. Where a good instrument has been set by the maker to a standard barometer, it is not wise to alter it frequently if it keeps in good working order for altitude measurements without being again set by a standard. On the other hand, however well the aneroid may have been made it works gradually to a slight change, caused by the smooth wearing of parts in action. It is well to have an aneroid, after one or two years' wear, cleaned and adjusted by the maker. It will then, if a good instrument, work well for many years.
836.—Directions for Measuring Altitudes.—Turn the outer rim of the instrument until the index carried thereby reads to the same point as the index hand. Raise the magnifier until the reading comes into sharp focus. Hold the instrument as nearly horizontal as possible, and tap the case lightly with the thumb-nail two or three times, so as to overcome any slight friction of its mechanism. This places the action of the works in equilibrium. Write down the observation as it now reads in the pocket-book, taking thousands from the right hand (large figures), hundreds from the right hand (small figures), tens from the lines to the left of this, and units from observation of the position of the index line in the space between the last and the next line. Say this observation reads 2465. Whether we ascend or descend, the instrument acts similarly. We will now presume we ascend to the height we require to ascertain, and take a second reading, 1945; the difference between these numbers, 2465 - 1945 = 520 feet, is the number of feet ascent. It is necessary, where exact measurement is required, to take the reverse reading, as the atmospheric pressure may have changed. We now descend, taking the last observation, 1945, and find the reading at the first position 2463 instead of 2465, that is 2 difference, which proves that the atmospheric pressure has decreased. If we take half this difference = 1 and correct the first deduction, 520 - 1 = 519 will give us the correct measurement, subject only in this instance to the irregular possible fall of atmospheric pressure, which will not in many instances, if the times of observation have been nearly equal, be a quantity worthy of consideration. It is not necessary to make any correction for the height of the observer in positions above ground, as the instrument must be placed at a uniform distance from the eye to obtain the reading. In mines it will frequently be necessary to measure the heights from the ground at which the observation is made.
837.—Various Improvements in the Aneroid.—It is uncertain whether any great internal improvements have been made in this instrument, except by Vidie, at various times. Many attempts have been made to increase the length of scale to obtain more open reading. These attempts have all been in the direction of increasing the difference of space between the fulcra of the levers or by additional gearwork, producing thereby a greater multiplication of the small unit of displacement of the axis of the vacuum chamber beyond the normal × 500, which is already great. The multiplication has been taken up to × 2000 or more. This increases the difficulty of manufacture and certainty of permanent action. Many of these plans were tried by Vidie and abandoned. A plan of Vidie's[61] of giving the hand three or four revolutions, and to register this upon a spiral scale upon the dial, also by counting on a second dial the number of revolutions, has been repeated with slight variation by E. T. Loseby in 1860[62] and by Major Watkin later. Vidie's plan of drawing back the hand to read the spiral has been modified also by Major Watkin in a manner which may be a little less frictional.[63]
Fig. 395.—Watkin's extended scale surveying aneroid.
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838.—Watkin's Extended Scale Aneroid.—This instrument is shown at Fig. 395, and has a very extended reading, consisting of three complete circles, in place of the usual single scale, with a hand or pointer sufficiently long to extend across them all. In order to show clearly which circle of scales should be read there is an indicator attached to the movement of the instrument which causes a series of figures (I., II., III., corresponding with the three circles) to be exhibited through an aperture in the dial. For instance, when the instrument is in its normal state the hand will point to the first or outer circle, and the figure I. will appear and remain in the aperture until the barometer falls to 27·8, where the break takes place in the circle, as will be seen in the illustration. The hand then takes up the reading on the second circle (where the break appears at 27·8) and figure II. replaces figure I. in the aperture, remaining there until the barometer falls to 25, when the reading is transferred to the third circle, and figure III. appears in the aperture.