84 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



twenty-fifth days, in which the temperature falls from 23 to 14, there is no 

 adequate recovery afterwards. It would seem if this were a dislocation within 

 the interferometer, though I noticed nothing differing from the usual be- 

 havior. A slight change of parallelism in the mirrors, due to drop of temper- 

 ature, is always first corrected. 







24 Z6 28 SO, 



It follows, therefore, that even if the temperature variations have here been 

 purposely exaggerated to exhibit the evidence more strikingly, there is very 

 little hope, in the case of the common metals, of using the permanent 

 torsional deflection produced by a weight for the measurement of variations 

 of gravitational acceleration. For the incidental and spurious variations of 

 dg/g amounting to over lo" 4 per day for the case of viscous yielding and 

 nearly 2 X icr 4 per degree of temperature for the case of rigidity, are too large 

 to be treated adequately as corrections. The above method of direct weighing, 

 even apart from its inconvenience, would not be more trustworthy, as it must 

 contain the same inherent errors. 



73. Further experiments with the preceding apparatus. The damping in 

 case of the apparatus, figures 104 and 105, when the plates p, p' are about 5X5 

 cm. square and within i or 2 mm. of the top and bottom of the shallow cases 

 c, c' is exceedingly good. The beam mm', even with the relatively thick steel 

 wire w used, is practically aperiodic. Hence, on putting a wide plate of brass 

 underneath the system pmm'p' and close to it for an additional protection 

 from air-currents, no further casing is needed. If the mirrors m, m' are wide, 



