The remaining portions of tlie cistern are the short glass cylinder 

 F, Figure 3, the two curved boxwood pieces l and /', and tlie kid 

 leather bag N, with adjusting screw O, clamps, etc. 



It is plainly seen that on turning the screw O, the leather bag may 

 be folded up into or withdrawn from the curved boxwood chamber 7, 

 in a manner to cause any desired change in the level of the mei-curial 

 surface. 



10. Ivory point. — At A, figure, is shown what is technically called 

 the " ivory point," which jn-ojects doAvnward from the top of the cis- 

 tern and forms a fixed and definite point, to which the level of the 

 mercury in the cistern can be adjusted in taking readings of the 

 barometer, as will be described hereafter. 



The ivory point is, therefore, the zero end of the scale, from which 

 all the measurements of the height of the column are made. 



11. Scale of haromefer. — The scale of the barometer is seen on the 

 left of the opening, at the top. It is most conveniently made of a 

 separate strip of metal, although sometimes it is engraved directly 

 on the metal tube itself. The length varies from about 4 inches, for 

 use at stations of only moderate elevation above sea level, to from 10 

 to 15 inches or more, for barometers intended to be used in balloons 

 or on lofty mountain summits. The graduations on the scale also 

 vary, being only 10 spaces to the inch in many instances and 20 in 

 others; the latter graduation is to be preferred on account of the 

 greater accuracy attainable in readings. 



The scale of the barometer when engraved on a separate strip is 

 attached to the metal tube by small screws in such a manner that it 

 may be adjusted slightly up and down, so that the 30-inch mark, for 

 example, of the graduations can be placed at exactly the right dis- 

 tance from the ivory point. This adjustment being made, the scale 

 should not be moved afterwards. 



12. Vernier. — A vernier is a device by which one is able to ascer- 

 tain accurately much smaller fractional subdivisions of a graduated 

 scale than could otherwise be observed by the eye without the aid 

 of a microscope. For example, with a scale having only 20 subdi- 

 visions to the inch a vernier enables us to ascertain accurately the one- 

 thousandth part of an inch. The name of the device is derived from 

 its inventor, Pierre Vernier. This portion of the barometer is the 

 little graduated scale C, Figures 1 and 2. 



A vernier consists, essentially, of a small graduated scale, the 

 spaces upon which are just a certain amount smaller or larger than 

 those on the main scale. Wlien two such scales are placed together 

 some particular line of the one will always be coincident, or very 

 nearly so, with a line on the other, and from this circumstance the 

 position of the zero line of the vernier in reference to the scale can 

 be veiry accurately determined, as will be readily understood from a 

 study of the following figures and explanation : 



Figure 4 exhibits the manner of graduating a vernier so as to sub- 

 divide the spaces upon the scale into tenths. In the figure, h is the 

 scale and a is the vernier. The lower edge of the vernier, which in 

 this case is also zero line, is exactly opposite or coincident with 

 30 on the scale. The tenth line on the vernier is coincident with 

 the ninth line above 30 ; that is, a space of 9 divisions on the scale is 

 divided into 10 spaces on the vernier, so that each space on the latter 



