476 



of tlie pressure from the oil-passages of tlie presstire-halance to the 

 tubes of the Leideu-manometers, in which coiuiiressed air is always 

 used for transuiitting the pressure. This trausunssion was at first 

 carried out by means of the steel (ul)e Z), with its level-gauge P,. 

 The level of th? oil in it could be easily kept up at the desired 

 height by the aid of the oil-pump O. P. When this arrangement 

 had been in use for some time, it appeared that small changes of 

 pressure in the oil of the pressure-balance, |)ioduced by the addition 

 of small weights on Ihe piston, were but very slowly and gradually 

 transmitted to the manometers M^^ and M^^^ ; it was therefore 

 desirable to transmit the pressure in the oil of the pressure-balance 

 to the mercury of the closed gauges by means of lubes exclusively 

 filled with liquid, eliminating all air connections. This arrangement 

 couhl be easily a|)plied to .l/i^o by screwing a steel tube with a 

 level-gauge P^ to the tap A',, (the object proper of which is to till 

 the manometer with mercury, when being mounted). Beyond this 

 gauge Pj a second gauge P, was mounted and the latter was in 

 connection with the oil-passages. Between the mercury in the lower 

 half of P, and the oil in the upper half of P, the pressure was 

 transmitted by means of glycerine. 



Our procedure was to bring up the pressure at first in the usual 

 manner with compressed air; if the stop-cock K^„ was then opened 

 and A'j. and A'^, closed, Ihe pressure-transmission exclusively by 

 means of liquids was realized. The pressure was further raised by 

 means of the oil-pump. This arrangement completely answered our 

 expectations : pressure-changes of ipooo i" ^'i6 oil of the pressure- 

 balance were now instantaneously indicated on il/i,„. 



3. Accuracif. An opinion as to the accuracy which may be ex- 

 pected may be formed by giving some data respecting the absolute 

 and relative accuracy of Ihe indications of the various instruments. 



The open manometer, when free of leakages and with a room- 

 temperature which is carefully kept constant, gives with certainly 

 an accuracy (absolute) of 0.01 7o- 



The manometer M^^, if the reading is certain to 0.1 mm. — which 

 is undoubtedly to be attained — guarantees 



at 20 atmospheres an accuracy of 0.008 " „ 

 „ t)0 „ „ „ " „ 0.020»,, 



For the manometer M ^^^ the following figures hold: 

 at 65 atmospheres an accuracy of 0.007 °/„ 

 „100 „ „ ,, „ 0.016 7„ 



