6 NBGRETTI AND ZAMBEA, HOLBOEN VIADUCT, B.C., 



suspend it on the brass bracket, its cistern passing through the ring at bottom, 

 and allow it to find its vertical position ; after which firmly clamp it by means 

 of the three clamping screws. 



Directions for taking an Observation. Having 

 taken the temperature by the attached thermometer, 

 the mercury in the cistern must be raised or lowered 

 by means of the thumb-screw (s), fig. 2, until the 

 ivory point (E), and its reflected image in the mer- 

 cury (D), are just in contact ; the vernier is then 

 moved by means of the milled head, until its lower 

 edge just excludes the light from the middle and 

 uppermost point of the mercurial column as seen in 

 fig. 5 ; the reading is then taken by means of the 

 scale on the limb and the vernier. In observing, the 

 eye should be placed in a right line with the fore and 

 back edges of the lower termination or edge of the 

 vernier. A small white reflector placed behind the 

 barometer will assist in throwing the light through 

 the brass frame and the glass tube ; and the observer's 

 vision may be further assisted by the use of a 

 magnifying lens. The great object in standard 

 barometers, is to obtain exact readings, which can only be done by having the 

 eye, the front of the zero edge of the vernier, the top of the mercurial column, 

 and the back of the vernier, in the same horizontal plane. 



To remove the Instrument. If it should be necessary to remove the 

 barometer, first, by means of the adjusting screw (s), fig. 2, drive the mercury to 

 the top of the tube, turning the screw gently when the mercury approaches the 

 top, and stop turning directly any resistance is experienced ; next remove the 

 instrument from the bracket, slowly invert it, and in carrying keep the cistern 

 end uppermost. 



3. The Barometer Vernier. The Vernier, an invaluable contrivance 

 for measuring small spaces, was invented by Peter Vernier, about the year 

 1630. The barometer scale is divided into inches and tenths. The vernier 

 enables us to accurately sub-divide the tenths into hundredths, and, even to 

 thousandths of an inch. It consists of a short scale made to pass along the 

 graduated fixed scale by a sliding or rack-and-pinion adjustment. 



The scales of standard barometers are usually divided into half-tenths, or 

 05, of an inch, as represented, in fig 6, by AB. The vernier, C D, is made equal 

 in length to twenty-four of these divisions, and divided into twenty-five equal 

 parts ; consequently one space on the scale is larger than one on the vernier, by 

 the twenty-fifth part of *05, which is '002 inch, so that such a vernier shows 



FIG 5. 



