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first rod L,, graduated over its whole length, and hanging between 
the limbs of the U-shaped manometer-tube A, was compared with 
the Standardmeter N°. 1 of the Physical Laboratory of Leiden, 
which according to its correction is at 7°; C. shorter by 0,005 mm. 
than the Métre des Archives. From this we calculate its length at 
0° C. 999.91 mm. The length between the marks 0 and 304 (cor- 
responding to 4 atm.) of this measuring rod was measured in two 
ways and found to be 303.976 e.m. and 303 971 em., which two 
values are in sufficient agreement. In order to determine the lengths 
of the other measuring rods Br—Byyy they were suspended together 
with the measured rod at short distances from each other, and at 
425 em. distance from the theodolites to be used for the reading 
of the mercury levels, the same distance at which they are used. 
The readings were made with telescopes rotating round perfectly ver- 
tical axes; during rotation I could not observe on the very sensitive levels 
a larger variation than 1/, mark, corresponding to !/2g mm. on the 
measuring tube. In four measurements the mean difference of the 
readings was less than 0.1 mm. 
The following corrections have to be applied in the measurements: 
A. The correction for the depression, mentioned above. 
B. The correction for the friction of the mercury, also discussed 
above. 
C. The corrections for the inclination of the telescope of the theodolite. 
But in the most unfavourable case, namely in the outer tube 
from 56—60 atm., this correction is only 1/99 of the vertical distance 
between the mercury meniscus and the level of the theodolite, and 
as in the measurements this distance was hardly ever more than 
3 cm. the correction may be neglected, especially for the other 
tubes where it is much less. 
D. The corrections arising from the temperature of the mercury. 
The temperature is measured by eight thermometers distributed 
over different parts of the apparatus. If the mean temperature is t, 
tables can be calculated for this correction from the formula 
A= L {1 — (a—k)t}, in which a is the co-efficient of cubic ex- 
pansion of mercury and k the co-efficient of linear expansion of 
brass. (The measuring rods are of brass). 
K. The corrections for the weight of the air which is compressed 
between the mercury of the successive manometer tubes. By means 
of the known values of the pressure belonging to each column, the 
specific weight of the air and the mean temperature, tables can also 
be calculated for these corrections. 
F. The corrections of the thermometers necessary for the corrections 
