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Physics. — J.C. ScHALKWwiJK: “Precise Isothermals. II. Accuracy 
of the measurement of pressure by means of the open mano- 
meter of KAMERLINGH ONNES.” (Communication N°. 70 from the 
Physical Laboratory at Leiden, by Prof. H. KAMERLINGH ONNES). 
§ 1. As the accuracy, which could be attained in the measure- 
ment of pressure, was of the greatest importance for attaining that 
high degree of accuracy which is required for my determinations of 
isothermals (Comp. Comm. N°. 67), I have made the measurements 
with the open manometer mentioned the subject of a separate inves- 
tigation. Although there is little reason to doubt the exactness of the 
observation, after the necessary corrections described in Comm. N°. 44 
have been applied yet some reason for doubt might exist in the fact 
that the correction for the depression of the mercury menisci, and also 
those resulting from the friction of the mercury in the capillary tubes 
are not sufficiently known. The former were borrowed from the tables 
of MENDELEJEFF and GuTKowsky ') and the latter were calculated by 
means of PoISEUILLE’s law; especially the latter give rise to many 
difficulties, as I could calculate only the mean bore of the mano- 
meter capillaries, whereas in PoISEUILLE’s *) equation the mean of 
the square of the bore is required. It is however a favourable 
circumstance that the variation of the level of the mercury in each 
of the manometer tubes hardly ever exceeds the amount of 1 c.m. 
an hour, and as in the narrowest of the capillaries this would give 
a difference of pressure of 0.02 c.m. per tube, with our degree of 
accuracy it may be neglected in comparison with a pressure of 
304 e.m. per tube. 
§ 2. The lengths of the tubular measuring rods (comp. Commu- 
nication NC. 44) which are of chief importance for the measure- 
ment of the pressure, were determined in the following way: the 
1) As the reading of the level and the top of the meniscus would double the time 
of observation the depression is already accounted for in the reading, by estimating 
and adding for each tube the required correction to the height. 
2) In order to test the use of the formula for tubes of the bore used (of 0.055 e.m. 
radius on an average), I have calculated from the velocity of the mercury through 
a similar tube the value of the co-efficient of friction , and found 0.00001725, 
differing only a little from the value 0.00001633 found by Warsure (0.01609 in 
the C.G.S. system). 
