When set properly, the vernier straight edge, the top of the mercury, and the observer's eye, should be on the same level; the edge (or pointer) just _touching_[36] the middle and uppermost point of the column. Great care should be taken to look thus square, or at right angles to the scale. Light, or something white, at the _back_ of the tube, assists in accurately setting the vernier, and may be shifted about to aid in reading off. * * * THE ANEROID has been recommended, in these pages, as a weather-glass; but it may increase its usefulness to append a table for measuring heights (approximately) by this, or any barometer, which can be compared with another, or itself, at a higher or lower station. If the measure of a height rather greater than the aneroid will commonly show, be required, it may be _re-set_ thus--When at the upper station (_within its range_), and having noted the reading carefully, touch the screw behind so as to bring back the hand a few inches (if the instrument will admit), then read off and start again. _Reverse the operation when descending._ This may add some inches of measure _approximately_. * * * * * In the following Table, the difference between the number of feet opposite the height of a barometer, at one station, and that at another station, is their approximate difference of height. TABLE. -----------+-----------++-----------+-----------++-----------+----------- Barometer | Height in || Barometer | Height in || Barometer | Height in Inches. | feet. || Inches. | feet. || Inches. | feet. -----------+-----------++-----------+-----------++-----------+----------- 31.0 | 0 || 26.8 | 3829 || 22.7 | 8201 30.9 | 85 || 26.7 | 3927 || 22.6 | 8317 30.8 | 170 || 26.6 | 4025 || 22.5 | 8434 30.7 | 255 || 26.5 | 4124 || 22.4 | 8551 30.6 | 341 || 26.4 | 4223 || 22.3 | 8669 30.5 | 427 || 26.3 | 4323 || 22.2 | 8787 30.4 | 513 || 26.2 | 4423 || 22.1 | 8906 30.3 | 600 || 26.1 | 4524 || 22.0 | 9025 30.2 | 687 || 26.0 | 4625 || 21.9 | 9145 30.1 | 774 || 25.9 | 4726 || 21.8 | 9266 30.0 | 862 || 25.8 | 4828 || 21.7 | 9388 29.9 | 950 || 25.7 | 4930 || 21.6 | 9510 29.8 | 1038 || 25.6 | 5033 || 21.5 | 9632 29.7 | 1126 || 25.5 | 5136 || 21.4 | 9755 29.6 | 1215 || 25.4 | 5240 || 21.3 | 9878 29.5 | 1304 || 25.3 | 5344 || 21.2 | 10002 29.4 | 1393 || 25.2 | 5448 || 21.1 | 10127 29.3 | 1482 || 25.1 | 5553 || 21.0 | 10253