112 



A. L. Day and R. B. Sosman — 



100° up to 1000°. This mode of procedure involved the assump- 

 tion that the agreement of the measurements made before and 

 after heating afforded adequate proof that no change had taken 

 place during heating. The justification for this assumption 

 lay in the fact that, (1) the' furnace was completely water- 

 jacketed to prevent any heat reaching the microscopes from 

 the furnace ; (2) suitable insulating material introduced between 

 the observer and the microscopes cut off any disturbing influ- 

 ence from the near approach of the observer's body; (3) the 

 microscopes themselves, and the carriages upon which they 

 were mounted, were connected by carefully selected invar bars 



Fig. 4. 



^ 



(//;/av/^— . — — ; /..:,,- '.. ' ■,■;■:'■.'■—, v — : ' [■' '.''/AWSSJ^Jl 



e mm ~~ 



in 



Fig. 4. Section through furnace showing bar, thermoelements (E, E) and 

 microscopes in position. A section through the arrow is shown in fig. 5. 



of negligible expansion coefficient, and, finally, (4) the faith- 

 ful agreement of all the measurements before and after the 

 many" heatings left no reason for suspecting a variation. 



Notwithstanding these conditions, it appeared to Chappuis 

 that some positive proof should be offered that the distance 

 between the cross-hairs remained unchanged while the heating 

 was going on, inasmuch as all the measurements were made 

 in terms of this distance. Accordingly, at his^ suggestion, it 

 was arranged to retain a standard unheated bar in the field of 

 the microscopes throughout the readings, so that the distance 

 between the cross-hairs would be subject to check at any time 



