( 307 ) 



diirinj; the upward inovoniPiif and open dm-inp: tlio downward 

 movenienf. 'I'lic npwar«l moNoniciit is liron^lit alK)n( lt\ means of 

 the thin wires /,. the downward niovrnKMit hv the w(M;i,ht of the 

 stirring a|)|>aralns itself wliieh for this pnrjiosc is weij^lited witli the 

 lieavv ring /„, by means of the rods x,,,. As yet a more rapid 

 motion of tlie stirring apparatus than this metiiod affords has not 

 been re(piircd ; if wanted a constinction with small rods instead of 

 lhrea<is wouhl be necessary. Tiie valves are hinged on bent pins 

 X,,^,. Tlie complete section of the stirrer to the right of plate II 

 shows the valves shut, the section of Xoi 'i' <''(" <op shows them 

 open. When the stirring apparatus is move<l nj) and down and 

 the. bnl)bles of vaj)our escape the movements of the valves resemble 

 those of the (ins of fishes. 



It is very important that the up and down motion of the ring 

 should be perfectly perpendicular and that the protecting cylinder 

 §, and the beaker B^^ should have a perfectly vertical position for, 

 to make the valves work properly, only a narrow space can be left 

 between the stirrer and the cylindrical walls. The cylinder $„ is 

 enclosed between two rings provided with grooves §i and §,, of 

 which the up})er is connected with the ring §5 by means of glass tubes. 

 Through the operation of the spring §31 and the arch Sj^, this ring 

 is pressed against the ring ^^ on to which the beaker i?„, with a 

 ground upper rim is fastened by means of cords. To this ring §, 

 the hooks |. are also fastened, against which the upper rim of 

 the beaker B^^ is also presseil by means of cords. In this way 

 a cylindiical space is reserved foi- the i)umping motion of the 

 stiri'er. 



h\ order to admit the measuring apparatus it was advisable to 

 leave free the whole space offered by tube ]^\, which is equal to 

 that in the bath available for a measuring apparatus. To this en<l 

 the threads Xi- formed of very thin silk cords enclosed in steel wire 

 are led through 3 openings 1\^ in the cover E of the bath and 

 then over a pulley axis Xa with three grooves to a connecting piece 

 X,. which is moved by a single thread passing (jver the pulleys x* 

 and Xs- riit' cord must be moved from outsi<le the case and the 

 case nuist remain ))erfectly air-tight. Tins is obtained by passing the 

 cord through an india rubber tube Xm- ^vhich at x«o fits hermetically 

 on to the cover of the cryostat and in which the thread x«i '^^ «il^o 

 heiiuetically fixed. A thin steel wire is wound spii-ally round the 

 india rubber tube. In this way the walls of the tid>e offer suflicient 

 resistance lo tlie almospheric pressure to prevent them fn»ni collapsing 

 when low pressure exists in the cryostat, while at the sanie time 



