EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 25 



of gravity or elsewhere can not be answered, as it is not unlikely that the 

 capillary forces introduced will in such a case be a serious consideration. 



The most interesting result obtained is the effect of temperature on the 

 inclined brace supporting the vertical standard of the horizontal pendulum. 

 It appears that even insignificant differences in the coefficient of expansion of 

 these parts are at once manifested as an appreciable change of inclination 

 varying with temperature. In fig. 14, for instance, if the large oscillations be 

 interpreted as a temperature effect, one may estimate that the change of i C. 

 of the temperature of the environment is equivalent to a deflection of x = 8 cm. 

 at the scale, or equivalent to about 01 = 2.5 seconds of change of inclination. 

 It is therefore necessary to avoid this deficiency of the apparatus with scrupu- 

 lous care ; in other words, to avoid all lateral bracing if the material can not 

 be guaranteed as rigorously homogeneous. Hence, in the more refined experi- 

 ments, the pendulum is to be swung with advantage from a single sufficiently 

 stiff metallic post anchored in the ground. Moreover, since the pendulum 

 with a trough for the float supported quite free from the pendulum is virtually 

 very light, a standard made of a length of i-inch gas-pipe well anchored in the 

 ground seems to be most promising. The case and the optical apparatus are 

 in every instance to be supported entirely free from the horizontal pendulum. 



The errors which have been detected in the case of the braced pendulum 

 are in all probability also present in the case of a pier, if the pier confronts 

 the illumination of the room or the heating-pipes of the building on one side 

 only. In such a case, the exposed side will expand on the cold side as an axis, 

 and a tilting of the pivotal line of the horizontal pendulum must result. Un- 

 fortunately, this is the condition to which the large pier in our laboratory is 

 subject and which it is impossible to remedy. It is probable that the excur- 

 sions observed with the steel pendulum in 3 are largely to be interpreted in 

 this way. Temperature observations will here be of little avail, since their 

 distribution in the immense mass of masonry is in question. Similarly, the 

 absorption and release of moisture when an unavoidably heated basement 

 room passes from the damp summer to the dry winter conditions may have 

 a similar tilting effect. 



15. The precision measurement of elongations. From another point of 

 view the exceedingly sensitive expansion apparatus which has been described 

 is interesting on its own account. In the diagram, fig. 16, the hvb triangle 

 supports the horizontal pendulum PP, normal to its plane, on the pivotal 

 hangers pp; g (outward from the plane of the figure) is the grating at the end 

 of the pendulum, n and m the concave mirrors of the displacement interfer- 

 ometer. Apart from the instrumental (elasticity and viscosity, etc.) and en- 

 vironmental conditions, such an apparatus should register expansions dl/l of 

 an order even smaller than 4Xio~ 10 per vanishing interference ring, for the 

 registered sensitiveness of the above apparatus could easily be increased. No 

 doubt much of this would be taken up by the yield of the apparatus; but 

 nevertheless it is over io 4 times smaller than the expansion of an average 



