( 37 ) 
correction resulting from the difference in level between the surface 
on which the pressure to be measured is applied, (3, and the top of 
the meniscus of the mercury in one of the manometer-tubes 0,, it 
is necessary to know the level of the mereury in the reservoir 
R,. This is indicated by the gauge-glass P, beside the reservoir Zs. 
It consists of a thick-walled tube, drawn out on both ends and 
provided with steel caps and steel capillary tubes t, fo, as described 
in Comm. n°. 44, which form at the higher and lower ends the 
connection with the mercury-reservoir Z (or what comes to the 
same with the siphon tube s3). The stopcock 4, serves to close the 
gauge-glass when the reservoir has been exhausted and we want to 
let mercury flow into it through the tube s3. 
The correction just mentioned for the vertical distance between the 
level of the mercury in the reservoir (as indicated by the gauge-glass) 
and the meniscus in the manometer where the reading is taken, which 
in the case of many manometrical measurements could but very 
roughly be applied, is accurately determined here with a kathetometer. 
A divided scale on which we can immediately transfer the rea- 
dings is placed at the side of the gauge-glass. With the aid of the 
kathetometer we compare the level of the graduations with the level 
of the graduations on the manometer-tubes to be read. This is done 
before and after they have been provided with the waterjackets mj, 
mg, mg and m4. To protect the observer the gauge-glass is provided 
at the back and on the sides with iron plates and in front with a 
thick plate-glass. 
On a board near &, stopcocks and tubings are fastened which 
are represented in Pl. I fig. 1 and 2 require no separate descrip- 
tion. By means of these we can: 
1°. close (by means of #9) the conduit s, through which the 
pressure is transferred!) by means of compressed gas ; 
20, read the pressure (kg being shut) transferred through this 
conduit, on an auxiliary manometer M, before the compressed gas 
is allowed to transfer the pressure into the measuring apparatus, 
in order to get to know, which manometer is to be connected; 
30, make and break arbitrarily the connection with the auxiliary 
manometer, whenever this seems desirable, while making the measu- 
rements of pressure ; 
1) As for instance a tube which forms a connection with an apparatus like that ot 
Dr. SrertsEma’s Comm. NO. 49, or a tube, which connects the manometers with the 
piezometer for gases compressed at low temparatures, mounted in another room. 
