( 37 ) 
paper was fastened behind the back window in the water-jacket a 
little above the meniscus. Here follows a reading on February 
2nd 1900. 
re | Position of C td 
reading. | level. eten dee 
Mark 1.8 (U — tube) 94.217 | + 2.25 94.218 (2.35) 
Meniscus (top.) 94.230 + 2.2 94.231 (2.35) 
Meniscus (level) 94.117 + 2.35 94.117 (2.35) 
Thermometer. t, | ta lina) renee A 
Reading. Ball OE | ee 8.08 | 8.04 
Corrected. S.04 8.03 | 8.00 
After the reading the mercury is drawn off through the cock at 
the lower end; to prevent the cock from being heated too much 
by the hand in doing so, we used paper as a means of insulation ; 
to take care that the mercury should break off each time in the 
same way, the surface of the mercury was just brought into contact 
with the point and then taken away again. 
The mercury was weighed on a balance which for 1 mg. 
moves over about 2 graduations and of which the ratio of the arms 
had been determined to within !/ ,000,000 ; the corrections of the weights 
were certain to within 1/0000 by means of repeated determinations. 
The correction for weighing in the air has been applied. 
The knowledge of the quantities of mercury drawn off each time 
is however by itself not sufficient to give the contents between the 
two menisci observed; for after each measurement the apparatus 
had to stand a long time while the water was being stirred, 
before we could expect that the mercury in the piezometer tube had 
assumed the temperature of the thermometers; in consequence the 
temperature sometimes showed important differences, especially in 
measurements made on different days 1). 
And so for each reading we must apply the correction for 
temperature to the entire quantity of mercury from the cock to the 
1) In one instance even the temperature on February 16th was 7°.24 C. and on 
February 20th 9°78 C. Owing to the small coefficient of expansion of the mercury 
it was not necessary to use the apparatus for the regular current of water of constant 
temperature. 
