( 222 ) 



reading 2 (or 4) menisci in lubes calculated for tlie liigliest ])ressure. 



The standard- manometer as a matter of' course is used to determine 

 pressures, that arc carefully kept constant for a long time, and by 

 prefeience by repeated readings from one and the same adjustment. 

 And so there was no reason for using the above-mentioned auxi- 

 liary means. 



5. The readw(). The position of the menisci is read on tubular 

 measuring rules, suspended between the manometer tubes in rings 



of Cardanus cq' ^1 , which rules besides promoting the 



accuracy of the measurements, hav<i the advantage of being easily 

 applied to the apparatus and removed from it. The measuring- 

 rule used with the manometer of type A in which adjustments are 

 made at different heights is a brass tube graduated in ra.m. over its 

 whole length. The measuring rules used with tubes of the type i? are 

 ordinary brass tubes to which only at both ends short graduated 

 scales u are fixed. We adjust at the menisci in the tubes B^ all of 

 which are at an elevation of nearly 1 M. or nearly 4 M. above the 

 ground with the aid of two theodolites. One of these is placed on 

 an isolated observation pillar in the room, 4 M. in front of the 

 wall against which the apparatus is fixed. Vertically above this 

 one a second theodolite is placed on a table (3.5 M. high) resting 

 on the same pillar. Next to this isolated elevation or observation- 

 tower stP|)S provided with a platform, such as are generally used 

 for observing purposes in the laboratory, are placed. 



In this our cafe it was very difficult to make the menisci clearly visible 

 in the way in which it is generally done viz. by sliding along the tubes 

 a piece of black paper tubing until the rim of this piece of tubing 

 almost touches the meniscus and the meniscus stands out as a dai'k 

 reflection against the bright background (white pa pei'), especially because 

 in order to protect the observer when he opens or shuts stopcocks, or 

 looks for leaks and his eye is very close to the apparatus, sheets of 

 plate glass P (fig. 1) have been placed in front of the manometer 

 receptacles. The sheets of plate glass are supported l)y clamps fixed 

 to beams r which are screwed on to the wooden strips / fixed every- 

 where along the walls of the laboratory for fastening purjjoses. 



Mr. J. C. Schalkwijk, assistant at the laboratory to whom 

 I am much indebted for ihe care bestowed upon the apparatus, 

 has hit upon the idea to place behind the meniscus a drawing of 

 black teeth against a white surface (comp. fig. 2 and 3) which is 

 reflected on the meniscus. The coming together of the reflection 

 of the teeth on the meniscus with the drawing of the teeth enables 

 us to determine accurately the place of the top of the meniscus. 



