_To Remove the Instrument when fixed to another Position._--If it should be necessary to remove the barometer,--first, by means of the adjusting screw, drive the mercury to the top of the tube, turning it gently when it is approaching the top, and cease directly any resistance is experienced; next, remove from the upper bracket or socket; lift the instrument and invert it, carrying it with its lower end upwards. _Directions for taking an Observation._--Before making an observation, the mercury in the cistern must be raised or lowered by means of the thumb-screw, F, until the ivory point, E, and its reflected image in the mercury, D, are just in contact; the vernier is then moved by means of the milled head, until its lower termination just excludes the light from the top of the mercurial column; the reading is then taken by means of the scale on the limb and the vernier. The vernier should be made to read upward in all barometers, unless for a special object, as this arrangement admits of the most exact setting. In observing, the eye should be placed in a right line with the fore and back edges of the lower termination of the vernier; and this line should be made to form a tangent to the apex of the mercurial column. A small reflector placed behind the vernier and moving with it, so as to assist in throwing the light through the back slit of the brass frame on to the glass tube, is advantageous; and the observer's vision may be further assisted by the aid of a reading lens. The object is, in these Standard Barometers, to obtain an exact reading, which can only be done by having the eye, the fore part of the zero edge of the vernier, the top of the mercurial column, and the back of the vernier, in the same horizontal plane. _Uniformity of Calibre._--The diameter of that part of the tube through which the oscillations of the mercury will take place is very carefully examined to insure uniformity of calibre, and only those tubes are used which are as nearly as possible of the same diameter throughout. The size of the bore should be marked on the frame of the barometer in tenths and hundredths of an inch. A correction due to capillary action, and depending on the size of the tube, must be applied to the readings. =5. Correction due to Capillarity.=--When an open tube of small bore is plunged into mercury, the fluid will not rise to the same level inside as it has outside. Hence, the effect of capillary action is to depress the mercurial column; and the more so the smaller the tube. The following table gives the correction for tubes in ordinary use:-- Diameter Depression, in Depression, in of tube. boiled tubes. unboiled tubes. INCH. INCH. INCH.