October 14, 1909] 



NA TURE 



457 



As stated by the makers, " the principle on which the 

 instrument works is the measuring of the strength of the 

 electrical currents which are constantly flowing between 

 earth and atmosphere, and which are always strongest in 

 the vicinity of subterranean water courses." It would be 

 interesting to know whether there is any scientific basis 

 for this statement. 



The writer has applied to the makers of the instrument 

 for particulars relating to it, but beyond sending him a 

 circular embodying the above quotations, he has been 

 imable to obtain from them much information. 



A. A. C.VMPBELL SWINTON. 



66 Victoria Street, London, S.W., October 7. 



MOVEMENTS OF THE EARTH'S SURFACE. 

 T N the Revue scientifique of August 2S is an in- 

 ■'■ teresting- address delivered by M. Ch. Lallemand 

 to the Association frangaise pour rAvancement des 

 Sciences. The address deals with two subjects : — (i) 



+ 10 



0" 6^ \Z*> I8b 



Fig. I. — Semi-diurnal oscillation of a pendulum under the action ol 

 ^^^ Observed wave. « . — . ™ . Calculated wave reduced 



Tides in the solid earth ; (2) gradual changes of level 

 in large tracts of the earth's surface. 



The first subject is introduced by a short historical 

 account of the attempts made in the past thirty years • 

 to discover alterations in the position of the vertical 

 relative to the earth's surface accompanying the 

 changes of direction of the sun and moon. This is 

 followed by a detailed account of the recent work of 

 Dr. Hecker, of Berlin. An illustration of his ap- 

 paratus is given, and an interesting account of the 

 manner in which, by a mechanical and optical device, 

 a horizontal pendulum, 0^25 m. in length, is made to 

 [jroduce effects such as could only be produced directly 

 by a vertical pendulum of length equal to the height 

 of Mont Blanc. 



Dr. Hecker's apparatus was placed in a chamber, 

 which was situated at a depth of 25 m. below the 

 surface of the earth, and kept at constant temperature 

 and humidity. The motions of the pendulum, greatly 

 magnified, were registered continuously on a revolving 

 drum. Roughly speaking, they amounted to a daily 

 oscillation of the vertical of about o'o2" north and 

 south. The greater part of this oscillation was 

 thermal in origin, being caused by the heating of the 

 upper layers of the earth's surface by the sun's rays. 

 It was possible to remove this term, and there was 

 left as a residual effect a semi-divirnal oscillation, 

 Avhich could be traced to the varying attraction of the [ 

 sun. More important, because it was more free from 

 thermal disturbance and greater in magnitude, was 

 the semi-diurnal oscillation of the pendulum, which 

 Dr. Hecker found corresponding to half a lunar day. 

 The close agreement between this observed oscilla- . 

 tion and a tfieoretical curve for the deviation is shown 

 in Fig. I. _ I 



Whereas the phase and direction of the changes in 

 the vertical agree closely with theory, the amplitude 

 of the observed change is much less than that which 

 theory indicates. Or, rather, we should say that the 

 amplitude is about 2/3 of that which would be j 

 observed if the earth were perfectly rigid. The j 

 difference between this factor 2/3 and unity is j 

 a measure of the extent to which the earth's 

 surface yields to the tidal force of the moon, and thus | 

 NO. 2085, VOL. 81] 



masks the deviation of the pendulum. It is interest- 

 ing to note that in 1884 Sir George Darwin, from 

 analysis of tidal records, found this factor to be o'6-jb, 

 also that his and Lord Kelvin's earlier estimate of the 

 rigidity of the earth accord closely with that now- 

 determined by Dr. Hecker, namely, 5/6 of the rigidity 

 of steel. The difference between observation and 

 theory is shown in Fig. 2. 



One interesting fact is illustrated in Fig. 2. The 

 reduction of amplitude from one curve to the other is 

 diflerent in different azimuths. Dr. Hecker has dis- 

 cussed this point, and shown it is in no way to be 

 accounted for as an indirect effect of accompanying 

 changes in the sea-level or the atmosphere. Whether 

 it is due to local surface conditions at Potsdam, or 

 whether it bears some relation to large structural 

 deformations of the earth, these are questions which 

 further research will alone elucidate. 



The second question discussed by M. Lallemand is 

 the examination of permanent or 

 ■ 10 gradual deformations of the earth's 

 crust. A short account of changes, 

 Q which have been shown to have been 

 caused by recent earthquakes, is fol- 

 j, lowed by a discussion of attempts 

 made in France to ascertain gradual 

 changes of level. Accurate work of 

 recent date has discredited Bourda- 

 loue's result that the sea-level at 

 Brest and Marseilles differs by a 

 result being ascribed to systematic 

 The difficulty of ascer- 



24 



f the moon, 

 in amplitude. 



metre, that 



errors in tlie observations 



taining permanent changes of level is increased by 

 secular alterations in the mean sea-level at the base of a 

 level-line. Added to this are the errors of the actual 

 work of levelling. M. Lallemand's estimate of the 

 error that would probably be introduced in ascertain- 

 ing the height of a hill-top 2000 m. above sea-level, 

 at a distance of 600 kiloin. from the sea-shore, is 



FjG. 2. — Daily apparent motion of the pendulum due to the 

 action of the moon. 



^^^^" Observed oscillation. 



. — . — Oljserved semi-diurnal oscillation. 



— — — — Semi-diurnal oscillation calculated for aiigid earth. 



12 cm. to 17 cm. even when. the levelling is done by 

 the most accurate mecnods at present available. In 

 view of the slowness with which changes of level take 

 place, an interval of at least thirty years ought to 

 elapse between successive levellings undertaken to 

 show changes of level. 



In time we may hope to ascertain by repeated 

 geodetical researches in what way countries, or even 

 whole continents, are rising and sinking. In such 

 work the geodesists will have the fullest support of all 



