5. This pressure of 14.7 pounds per square inch is, in the main, 

 nothing more than the weight of an air column having a sectional 

 area of 1 square inch and extending vertically to the upper limits of 

 the atmosphere. In addition to the weight, pure and simple, how- 

 ever, such influences as the wind, the rapid heating and cooling 

 in confined layers of air, and other causes modify by small amounts 

 the elastic pressure of the air. 



6. Other forms of barometers. — Within the past 50 or 60 years a 

 form of barometer, made entirely of metal, has been devised, and is 

 widely used at sea and by tourists and others on account of its con- 

 venience and portability. This form is commonly designated the 

 aneroid, a word which signifies " containing no liquid." The in- 

 strument is also often called the holosteric barometer, meaning 

 " wholly of solids." Aneroids, though often highly sensitive, are, 

 at best, much less accurate than properly constructed mercurial 

 barometers, as will be explained later. 



I. Mercueial. Barometers 



7. Fortin harometers^ Weather Bureau pattern. — In order that the 

 height of the mercurial column may represent accurately the true 

 pressure of the air, and in order to detect the comparatively small 

 changes of pressure from day to day, many refinements are necessary 

 in the construction of the instrument and great precision of measure- 

 ment is required. An excellent form of the mercurial barometer, 

 satisfying the requirements just stated, was devised by Fortin, and is 

 now very widely used the world over. The particular pattern used 

 by the Weather Bureau, sometimes called the Green barometer, is 

 figured on page 4. 



8. The barometer consists of a glass tube, about 14-inch inside 

 diameter, closed at the top and inclosed in a thin metal tube, through 

 which large openings are cut on opposite sides, exposing to view the 

 glass tube and mercurial column. The graduated scale is formed 

 at one side of this opening, and a short tube or sleeve, also gradu- 

 ated (shown at C, figs. 1 and 2), encircles the barometer tube and 

 slides smoothly within the metal part, motion being given to it by 

 means of the milled head D, and a small rack and pinion inside. 



At E, Figure 2, is shown what is called the attached thermometer. 

 The bulb of this is entirely concealed within the metal tube, and is 

 between it and the glass barometer tube, so as to show as nearly as 

 possible the mean temperature of both the brass tube and the mer- 

 cury. 



9. Cistern. — The special feature of the barometer is a cistern so 

 constructed that the level of the mercury within may be changed 

 greatly and adjusted to a fixed index point. 



The topmost portion of the cistern consists of a small boxwood 

 piece G, Figure 3. The glass tube t passes through the central por- 

 tion of this, to which it is secured by a piece of soft kid leather 

 folded in a peculiar manner and securely wrapped to both the glass 

 tube and the boxwood cap G. The flexible joint thus formed will 

 not allow the mercury to escape, but permits the passage of air to 

 and from the cistern. 



