( 219 ) 



The tube sliown in fig. 1, sorves to adjust at positive and negative 

 differences to within V2 atmosphere. 



§ 3. The connections and the stop coch hoard. In tlie appnialus 

 represented liero, tube A at one end has eonfaet with tlie atmospheie 

 by means of a drying tube Q, coated with P^ O5 by which it is protected 

 from moisture. And so in this ease an india-rubber tube is suffi- 

 cient connection. At the other end however the ordinary contrivance 

 which is found at both ends of all other separate manometers has 

 been fjistcned. To the tliiek-walled glass capillary (comp. fig. 4) p, a 

 steel capillary </ has been cemented which carries an overpipe U' 

 with a nut U. This nut is screwed (comp. fig. 1) on the eorrespon- 

 diug' tap (of 7"i) of the stopcock board; the steel capillaiy gives 

 elasticity to this joint. The communication between the other 

 manometers is also obtained by screwing the connection-nut on to 

 the T-picces, 'l\ on the stop-cock board. The horizontal branches 

 of the T-pieces form the connection between the successive 

 manometers, the steel stop-cocks in the veitical branches of the 

 T-pieecs form the communication of the sjiace between the lower 

 meniscus in one manometer and the upper meniscus in the next 

 iranometer with the main feeding tube //, from which when brought 

 lo a sufficient pressure, gas can be admitted in that space in oi'der 

 to fipply the desiieil pressure on the columns of mercury. 



This main feeding tube (of copper with brazed on branches) 

 is filled by the bronze stopcock X through the steel cioss- piece N 

 from a cylinder with gas compressed at a sufficient piessure — dry 

 air or oxygen or hydrogen, for lower pressures sometimes diy 

 carbonic acid — which gas passes over P2O5 (in the drying tube 

 J>) before entering the apparatus. The cross piece A' enables us at the 

 same time to make communication between the maii()mct(!r and the 

 apparatus in which we want to measure the pressure; such an apparatus 

 in the drawing being a metal-manometer as repiesented by M. This 

 metal-manometer and the stop-cock Z are also used to measure the 

 pressure in an apparatus which is joined to ^, before causing com- 

 munication between this apparatus and the open manometer by means 

 of the stop-cock Y. 



4. The adjustment at a given pressure. The admission of the 

 compressed gas in the diflerent spaces in the manomelers must be 

 performed with the greatest caution. In the first place we have to 

 prevent the overflowing of mercury from one manometer into the 

 other; we have alluded already to the disagreeable consequences of 

 this. Then we must remark that when we cause mercury to rise in 

 one of the manometer-tabes -ög f. i. and admit tliei'efore (keeping the 



