

It: 



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W 



T 



17. Tuch cistern. — Many of the barometers of the Weather Bureau 

 are fitted with an improved form of 

 cistern, devised by Charles B. Tuch, 

 formerly of the instrument division. 

 The construction of this is shown in 

 Figure 9. 



The chamber for the mercury is formed 

 of the iron cylinders c, A;, provided with 

 windows at the top, and a small glass 

 cylinder /. The glass barometer tube is 

 fastened into a metal piece &, by means 

 of several thicknesses of leather washers, 

 held and clamped by a screw e, above. 

 The piston o fits the cylinder very snugly 

 and can be moved up and down by 

 means of the milled-head screw W, 

 thereby adjusting the mercury to any 

 desired level. 



18. Fixed cistern harometers.-^lt is 

 very evident that as the column of mer- 

 cury rises and falls in a barometer tube 

 there is a corresponding change in the 

 level of the surface in the cistern, and as 

 long as the quantity of mercury in the 

 whole barometer remains the same, it 

 follows, except for slight temperature 

 effects, that the true height of the column 

 of mercury may always be found simply 

 from readings at the top end, a due al- 

 lowance being made for the slight rise 

 and fall in the cistern. 



Adjustable cistern barometers, such as 

 described in paragraphs 9 and 17, are in 

 general the most accurate, as the correc- 

 tion for capillarity, paragraph 26, is 

 usually more constant, and the accidental 

 escape of a little mercury from the cistern 

 does not matter. Still, very accurate re- 

 sults may be obtained by the use of well- 

 made barometers with fixed cisterns, and 

 this form is often adopted in the con- 

 struction of barometers for use on ship- 

 board. 



The relation between the true length of 

 the column and the observed position of 

 its top depends upon the relation between 

 the inside areas of the cistern and ba- 

 rometer tube. When this relation has 

 been once worked out it is then neces- 

 sary in reading the barometer to observe 

 only the position of the top of the column 

 and apply a " correction for capacity.*' 

 (See par. 24.) 



311681°— 41 2 



I J" 



Figure 9. — Tuch barometer cistern 



