High-class instruments, such as that here described, yield _exact_ readings; but, in order to note them accurately, it is important that the eye, the zero edge of the vernier, the top of the mercurial column, and the back of the vernier should be in the same horizontal plane; conditions which may be obtained after some practice. The accompanying illustration shows a form of barometer which, though not much used in this country, is deservedly popular on the Continent as a standard station barometer. It is called a Syphon Barometer, and was designed by Gay-Lussac. The open end of the tube is bent up in the form of a syphon, the short limb being from six to eight inches long; it is furnished with metal scales and verniers, and is mounted on a mahogany board with attached thermometer. These barometers require no correction for capillarity or capacity, each surface of mercury being equally depressed by capillary attraction, and the quantity of mercury falling from the long limb occupies the same space in the short limb. The usual correction for temperature must, however, be applied. A scale of inches, measured from a zero point taken near the bend of the tube, furnishes the means of measuring the long and short columns. The difference of readings is the height of the barometer. The VERNIER is a movable scale for subdividing parts of a fixed scale, and was first applied to that purpose by its inventor, M. PIERRE VERNIER, in 1630. In the barometer the parts to be divided are inches, which by the aid of this invention are subdivided into 10ths, 100ths, and 1000ths. Fig. 27 shows the scale of a standard barometer divided into 1/2-10ths, or ·05 of an inch. The Vernier C D is made equal to 24 of such divisions, and is divided into 25 equal parts, from whence it follows that one division on the scale is 1/25th of ·05 larger than one on the vernier, so that it shows a difference of ·002 of an inch. The vernier reads ·0, or zero, upwards; D, therefore, indicates the top of the mercurial column. [Illustration: 26. Syphon Barometer. Scale about 1/12.]