(29) 
lower end is estimated only a little too low, the depression become® 
too small and hence the mercury height read too large. 
This systematic error amounts only to of the pressure 
l 
16000 
measured and moreover will only have any influence at pressures 
above 32 atm. (for then only the tubes of the second system are 
used). By reading the real height of the menisci perhaps even this 
slight deviation might be prevented. 
Physics. — J.C. SCHALKWIJK: “Precise [sothermals, III. A water- 
jacket of constant ordinary temperature.” (Communication 
N°, 70, (continued) from the Physical Laboratory at Leiden, 
by Prof. H. KAMERLINGH ONNES). 
S 1. The necessity of having at disposal a current of water at 
a constant temperature. In consideration of the small heat conduc- 
tivity of gases and the great thickness of the walls of piezometer 
tubes, used in my experiments (comp. Communication N°. 50 of prof. 
H. KAMERLINGH ONNes, June 24th 1899) it is desirable to take care 
that the temperature of the surrounding water cannot vary more 
than some hundredths of a degree per hour, and that it can be 
accurately adjusted at the desired value and be kept almost con- 
stant during 5 hours. This offers however many difficulties; for 
the bath must be more than 80 c.m. high, and we must be able 
to read accurately the marks over the whole length of the tube, 
which excludes coating the bath with a badly conducting substance 
as a protection. Hence there will be a continual large loss of heat 
from the surface of the bath. The distance of the piezometertube 
from the glass wall must be very small with a view to the refraction 
of the rays emerging from the water, as we must be able to read 
very accurately the difference in the height of the mercury in the 
piezometer tube C, (comp. the plate Proceedings June 24th 1899) 
and in the measuring glass P *); therefore 6 c.m. was taken as 
') A means used by Dr. N. Quixr (comp. his thesis for the doctorate 1900, p. 15 
and fig. 5) cannot be used in our case. He placed the observation-tube in a rectan- 
gular basin containing about 40 liters of water and kept up a constant temperature 
by means of two liquid-resistances (saturated ammonium chloride) carrying an alternating 
current, an exterior kriippin resistance to be regulated by hand and rotating bladed 
wheel. Moreover in accurate determinations of isothermals, the measurements proper 
occupy each time about three hours and as therefore many readings have to be made, 
the regulation of the kriippin-resistance would cause an undesirable interruption of 
the series of observations, 
