( 35 ) 
It is obvious that changes in the temperature of the room may have 
a considerable influence on the constancy of the temperature in the 
observing bath: although I could not notice in the heating bath a 
variation of 0.1 deg., it sometimes happened with a great change in 
the temperature of the room that the temperature in the observing bath- 
changed some hundredths of a degree. ‘Therefore it is desirable to 
keep the temperature of the room as constant as possible. 
The results obtained with the apparatus described were quite 
satisfactory; although under some circumstances we could observe 
variations of almost 0.1 deg. C. in the heating bath, they were not 
perceptible in the observing bath. 
Here follow some readings made during my observations. In the 
first place I give an instance of the adjustment for a case when 
sufficient care had not been taken in the regulation of the constant 
flame, so that the regulating flame was sometimes extinguished. 
This happened on July 7. 
3.59 
| 
7 July | time 2.29 | 2.47 | 3.09 | 3.24 3.44 
199.763 
temp. | 190.75 rde ho ee 19°. 80 
The accuracy of adjustment is insufficient for my purpose. But 
if the required precautions are taken, the temperature is accurately 
maintained to 0.01 deg. C. as appears from the following observa- 
tions on two days: 
25 Aug. | time 2.48 3.09 | ss 3.28 3.48 4.07 4.26 
temp. | 190.785 | 190.785 | Wen | 199.78° | 149,78 p 190.050 
27 Aug. | time 2.44 9.56. | 3.07 |. 3-98 3.36 3.48 | 
mn daden 190.785 | (19°. 788") 19°. 78%) |, 198.800, MTD SRASR 
Physics. — J. C. ScHALKWIJK: “Precise Isothermals. IV. The 
calibration of piézometertubes.” (Communication N°. 70 (274 
continuation) from the Physical Laboratory at Leiden, by 
Prof. H. KAMERLINGH ONNES). 
In this paper the method for the calibration of piézometer tubes, 
mentioned in Communication N°. 50 is described more in detail '). 
1) AmaGaT mentions only that he calibrated carefully, but not how and to what 
degree of accuracy; a source of uncertainty in the calibrations of RreNaurr is men- 
tioned and corrected by Lepuc, who also employed constant temperature and consi- 
dered the volume of the meniscus in the calibration of his bulbs; Bunsen calibrated 
by admitting small measured quantities of mercury. 
3* 
