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cury ia much greater than that of gas, one might take the capilUiry 

 through which ouly gas flows, narrower than tliat through which 

 mercury flows, without causing thereby considerable delay in adjust- 

 ment. As the dimensions of those capillary tubes however do not offer 

 any difficulties, aud it is not desirable to take them narrower than 

 is stiictly necessary (comp. § 7) I used capillaries of nearly the same 

 section both for gas and mercury. 



The section of the capillary tubes is from 0,75 m.m^. to 1.37 m.nr. 

 The length of the upper cylindrical vessel is 8 cm. This length 

 leaves more than sufficient room in adjusting the meniscus, without 

 the accurate determination of the height of the mercury being damaged. 

 The cylinder may not be shorter because when the mercury rises, 

 danger might arise for the mercury to overflow which would cause 

 all the other manometers to overflow likewise. The whole apparatus 

 would temporarily become unfit for use and even the joints on the 

 board to which the stopcocks are fixed might be endangered. 



The length of the lower cylinders is calculated after the available 

 capillaries aud the upper cylinders. It has been taken into account 

 that when the lower cylinder contains so much mercury as necessary 

 to fill the whole capillary and the whole upper cylinder, a space 

 of 4 — 5 cm. long remains above the mercury. If this precaution 

 had not been taken, gas bubbles that might rise (in blowing off 

 the manometers) would cause the mercury to overflow, which as 

 mentioned above must be prevented carefully. 



An open manometer cannot consist of tubes of the type B only 

 as in these we can only adjust at a fixed difference of pressure 

 (position of the mercury nearly in the middle of the upper cylinder). 

 With n of these tubes, each measuring a difference of pressure of »■ 

 atmospheres one can adjust accurately at nr atmospheres only. For 

 adjustment at the additional pressure above nr atmospheres we want 

 another tube, viz. the tube ^, the first in the plate. 



In constructing shortened manometers we can very succesfully 

 avail ourselves of tubes of the third type, C (comp. fig. 1). These 

 are distinguished from the type A by their having the two branches 

 of the U equal in length so that we may adjust both positive and 

 negative diiferences of pressure, according as we adjust either at 

 rj on the left and rj on the right or conversely at r-^ on the right 

 and at rg on the left. Suppose that we have adjusted the tubes 

 A and B-^ to B^ together at 35 atmospheres and that we want to 

 measure the pressure of 34.8 atmospheres, this can be obtained, (the 

 adjustment in all preceding tubes remaining unaltered), by adjusting 

 the mercury in tube C at — 0.2 atmospheres difference of pressure. 



