(221 ) 



In orrlor to stick to the principles of the coustruction nf our ap- 

 paratus we should have at hand C tubes of different sections and we 

 should always have to join on such a tube as agrees most with the 

 pressure to be measured. 



From what preceeds it appears that when we have adjusteil at a 

 certain pressure and we want to pass to another this can be done 

 easiest by 4 atm. at a time. In order to rise 4 atmospheres we must 

 lot the last tube S^ go back to the mark ^Tu by drawing off' through 

 A'yJ, shut K A and K^^ and admit gas through X until the required 

 adjustment in B^o is attained. In order to let the mercury fall 4 

 atra. we must act conversely, viz. draw off through KA till the 

 mercury in B^^ has fallen; shut KA and open isTjo and again admit 

 gas through X until the mercury in -Bn has reached the middle of 

 the upper cylinder. 



If we want to pass over to pressures higher by a fraction of 4 

 atm. say 2 atm. then we must proceed more cautiously; as a rule one 

 should work with all stop-cocks shut, because all menisci will vary 

 at a higher adjustment in A, but it will often bo sufficient to bring 

 A almost to the desired new pressure through the stopcock A'j and 

 to bring the other menisci to a proper position by admitting a higher 

 pressure through A', if only we have left unimpaired the reserve 

 space in the upper cylinders. In none of these cases the adjust- 

 ment will offer any difficulties. 



It is however more difficult to let the pressure fall by differences 

 of less than 4 atm. We then must first adjust at 4 atmospheres lower 

 nnd must raise the pressure to the desired pressure by adding the 

 remaining difference, if the position of the mercury in tube A per- 

 mits. If not, we shall have to empty the main tube, to blow off" the 

 manometers, and to raise the pressure anew from to tiie desired 

 pressure. For in the tube A we can only adjust at a rising j)ressure, 

 because at a falling pressure mercury would overflow from the upper 

 receptacle Bj. 



If we should want an apparatus in which we can adjust with 

 the same rapidity at rising as at falling pressures, this should 

 consist entirely of A tubes and we should have to read the addi- 

 tional pressure above the multiple of 4 atmospheres on the tube 

 containing the highest pressure. Or we should have to add to a 

 manometer consisting of Jy-tubes a differential tube C of 2 atmospheres, 

 or two tubes C of 1 atmosphere each to which as in the case of 

 the separate manometers a special supply-stopcock has been joined. 

 The remarks made previously concerning the diameter apply to 

 t;hese tubes unless one should be satisfied for all pressures with 



