( 8G ) 



The pressure-cylinders of the two manometers are conneeted at 

 their upper ends with an hydraulic pump; from the latter and from 

 the tube h they are separated by cocks, so that each of them may 

 be put out of use separately. By means of the hydraulic pump the 

 mercury may be forced from one of the pressure-cylinders into the 

 capillary tubes //, g and ƒ, and so at the required height into D. 



The difference in height of the mercury in D and in the mano- 

 meters is taken into account as a correction for the pressure. 



The volume of D is so large that after a change of pressure the 

 mercury- meniscus in it always remains visible, without the hydraulic 

 pump being required continually. 



§ 5. The water-bath. 



A constant temperature is obtained by causing water to flow 

 through the water-bath in which the apparatus is placed. Whenever 

 vrater from the main was used the temperature was regulated by 

 means of the same apparatus which was used by Dr. van Eldik 

 in his investigations on capillary ascension i). 



If the temperature of the water from the main was higher than 

 was required for the observations it was cooled by means of ice 

 and a circulation was brought about, on the whole agreeing with 

 that used by Dr. de Haas in his researches on the viscosity of liquids 2). 



The water-bath is made so that the whole apparatus may be put 

 into it from above. For this the four cocks Z?i, /ij, ()), C'2 are mounted 

 on a base-plate H, to which the cover K of the water-bath is 

 fastened. In the centre of this cover is an opening to let thi'ough 

 the tubes e and ƒ; a rim surrounding that opening serves to fasten 

 a glass tube L in it. Besides in the four corners of the cover are 

 smaller openings in which the tubes b' and c', the other end of 

 and a thermometer are tightly fixed by means of corks. At the 

 lower part of the base-plate is screwed an anglepiece with sliding 

 tube supporting the reservoir. 



The water-bath itself consists of a rectangular box M, containing 

 the four cocks and a cylinder i\^, surrounding the reservoir. In the 

 front of the box are four openings with necks corresponding to the 

 steel pins of the cocks ; when these have been placed into the water- 

 bath we apply brass lengthening-pieces to those pins and the ope- 

 nings are shut water-tight by means of corks. The double cock A 

 has a water-bath of its own soldered to it; an india-rubber ring 

 P (high 10 cm.), which can slide along the cylinder, and when it is 



1) VAN Eldik, 1. c. p. 32. Comm. etc. N". 39. 

 ') DE Haas. Dis. Leiden 189é, p. 8. 



