On the Rigidity of the Earth. 481 



It was at once observed, however, that this same condition 

 existed in the case of the exciting radiations from the mercury 

 arc, on account of the great density of the mercury vapour 

 in the red-hot quartz tube. The mercury lamp was accord- 

 ingly extinguished and allowed to cool off. On relighting 

 the lamp, the fluorescence of the iodine vapour was at first 

 too feeble to permit o£ observation with the echelon, but as 

 it gradually brightened, as the temperature and intensity of: 

 the mercury lamp increased, the lines appeared, and were 

 observed distinctly narrower than the distance between the 

 orders. Moreover, their width appeared to be the same as 

 that of the exciting lines of the arc, which came out much 

 stronger than the resonance lines on account of diffused 

 light. As the arc lines widened, the resonance lines increased 

 in width at the same rate, until they fused into a continuous 

 band, due to the overlapping of orders. Calculation showed 

 that the width of the lines when they were first observable 

 in the echelon was of the order of l/20ch of the distance 

 between the D lines. It seems probable that, by still further 

 improving the conditions, a more extended study of resonance 

 spectra with the echelon can be made. To do this it will be 

 necessary to excite the fluorescence by a low temperature 

 mercury arc burning in glass. Probably a long iodine tube 

 passing along the axis of a Cooper-Hewitt lamp, so as to be 

 completely enveloped by the arc except at the ends, would 

 give the desired results. 



T have succeeded in obtaining resonance spectra from 

 iodine vapour with other monochromatic excitations, but 

 have not vet studied them. 



XL VIII. On the Rigidity of the Earth. By J. Prescott, 

 M.A., Lecturer in Mathematics at the Manchester School of 

 Technology *. 



r|^HE interest in the question of the rigidity of the earth 

 X has been revived in recent years by the careEul and 

 convincing experiments of Dr. Hecker of the Prussian 

 Geodetic Institute f . Dr. Hecker placed two horizontal 

 pendulums at right angles to each other and introduced 

 devices for recording the motion of the pendulums for a 

 long period of time. The motions of the two pendulums 

 are the same as the component motions of the bob of a 



* Communicated "by the Author. 



t Beobachtunqen an Horizontalpendeln,des k. Preussischen Geodiltischen, 

 Institute. Neue Folge, N. 32 (1^07). 



