EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 21 



of <p gave a = o.$x seconds of arc. These (summer) data are given in fig. 14 

 and the temperatures are inserted in the same figure. The work was continued 

 for about 6 weeks, not all of the data finding room in the figure, and the graph 

 after July 3 had to be displaced, as shown. 



The new results still partake of the same tendency to enormous variations 

 which characterize the older (winter) data. The essential error has, therefore, 

 not been removed. On comparison with the detailed temperature curve above, 

 however, the clue of the anomaly is obtained, for although the temperature 

 variations are not quite contemporaneous with those of inclination, there can 

 be no doubt of the immediate relation between them. The case is all the more 



FIG. 14. 



puzzling, however, as single degrees are in question, enormous changes of 

 inclination being produced by 4 degrees. Under the circumstances, moreover, 

 complete identity in the direction of variation of temperature and inclination 

 graphs was not to be expected, for the temperatures given are those of the 

 water in the float and will therefore vary more sluggishly than the tempera- 

 ture of the metal parts. The air temperatures, again, which were also taken, 

 would vary faster than those of the metal, evidence for which will presently 

 be shown. It is therefore next in order to actually examine the structure of 

 the standard of the horizontal pendulum. 



13. Effect of temperature on the scaffolding. To give the columnar sup- 

 port of the horizontal pendulum adequate steadiness, it was braced from be- 

 hind as shown in fig. 15. It was not foreseen that any menace could lurk in 

 such a system, such as was later detected. In fig. 15, ABC is a side-view of 

 the brass vertical standard (in duplicate, as shown in fig. 7), the horizontal 



