ing bar (wo thermometers B and C, tlie last two about a metre from 

 each extremity. Moreover there were placed in the comparator case 

 a registering thermometer D, two thermometers E and F upon the 

 outside of the aluminium case about above the thermometers B and C, 

 and two thermometers G and //at the two extremities of the comparator, 

 which were read through glass-covered openings in the end-walls. 



On the whole the temperature readings were of such a nature, 

 that there is every reason to believe that the readings of the thermo- 

 meters A, B and C may be taken as the temperatures of the metre 

 and the measuring bar. 



4. Programme of the measurements. It was arranged, that the 

 measurements should be made by the two engineers of the Govern- 

 ment Commission for triangulation and levelling, A. Wildeboer and 

 J. W. DiEPERiNK, and a member of the commission, H. G. v. d. Sande 

 Bakhuyzen. Each of these made a complete series of measurements. 

 Mr. Wildeboer and Mr. Dieperink arranged everything beforehand, 

 so that (1) metre and measuring bar were parallel to each other 

 and at the same level, (2) the micrometer wires were parallel to the 

 division lines and showed no parallax witli regard to the division 

 lines, (3) in the extreme positions of the carrier the division lines of 

 the meti'e and of the measuring bar appeai'cd in the correct position 

 in the field of the microscopes. In the adjustment of the level of 

 metre and bar, so that no parallax could be detected of the micro- 

 meter-wires with respect to tlie division-lines, the adjustment of one 

 of tlie observers was always checked by a second or third. 



When the carrier had been placed in one of the extreme positions 

 and the microscopes were therefore directed upon the end lines, say 

 of the metre, the observer placed the micrometerwires of the left- 

 hand microscope twice in succession upon the line, then took four 

 readings with the right-hand microscope, and finally two with the 

 left-hand one. In the middle of these eight readings the thermo- 

 meter on the meire was read. The carrier was now brought into 

 the other extreme position, so that portion a of the measuring bar 

 came under the microscope. In the same way as for the metre, 8 

 readings were taken with the microscopes, and readings of the 

 thermometers. The observer then returned to the metre, and in the 

 same way took seven sets of observations in succession, alternately 

 upon the metre and the selected portion of the measuring-bar, each 

 consisting of 8 readings. 



A series of observations of this kind, which lasted about a quarter 

 of an hojir, we shall henceforth call an observation-series. 



