342 



KNOWLEDGE. 



September, 1910. 



If, instead of thinking of the behaviour of a phim- 

 met on a rigid earth we pass to the other extreme and 

 think of the earth as a perfect fluid responding sensi- 

 tively to the least stress, and if we suppose our pen- 

 dulum to be on a raft iloating on the surface of this iluid 

 Earth, then it would be quite impossible to obtain 

 any evidence of the existence of the forces shewn in 

 Figure 4. The pendulum would ne\er move at all : 

 the lunar drag on it and the tilting of the raft on 

 the deformed Earth would precisely cancel each other. 

 This is a very difficult point, but the truth of it can 

 be seen in a general way, if one remembers that the 

 plumb line and the surface of the fluid are certain 

 to be perpendicular to each other. If some force 

 displaces the one it will displace the other in perfect 

 harmony. Many examples of this occur in surve\'ing 

 work. Near any steep high hill the {)lumb-line 

 is appreciably diverted, but 

 so is the surface of the sea o.o/o 



or of a mercury bath. The 

 plumb-line and the surface of 

 a perfect fluid \\\\\ ahcays be 

 perpendicular to each other, / 

 and for this reason the instru- t 

 ment on the raft would record \ 

 no movement of the plummet 

 over the base. And for the 

 same reason there would be no 

 movement of the water on the 

 coast, if the coast responded as 

 perfectly to the solicitations 

 of the Moon as did the water. 

 There would then be no tides 



in the oceans of an Earth of which the rocks acted 

 like a perfect fluid. 



From these general considerations, and without 

 going into elaborate mathematics, it can be seen 

 from the fact that the pendulum recorded less than 

 theory predicted for a rigid Earth that some }ielding 

 was taking place in the rocks on uhich the instru- 

 ment was based. The amplitude of the recorded 

 oscillations was about two-thirds of that expected 

 for a rigid Earth. This gives all that is needed to 

 calculate the elasticity of the crust, and the result is 

 that it must be about as elastic as steel. A bedrock 

 of several thousand miles of solid steel will be 

 regarded by engineers as a good enough base for any 

 structure that they are called upon to build ; so we 

 need not be nervous when \\e hear that we are being 

 raised and lowered by the elastic yielding of such 

 foundations. The elongation shewn in Figure 1 

 would amount for a fluid Earth to something like a 

 yard ; for a steel Earth it would be about a foot. So 

 if we assume that the water of the ocean acts as a 

 perfect fluid and the crust acts as steel would, then 

 we can see that the oceanic tides will rise and fall 

 about two feet on the coast instead of three feet ; 

 and it is now generally held that the tides are 

 diminished by about a third in consequence of the 

 yielding of the crust. 



A better conception of the completeness of Dr. 



Hecker's work will be gathered from an examination 

 of the next two diagrams. Just as the Sun \'isits the 

 Northern and Southern Hemispheres alternateK'. 

 producing winter and summer, so also each month 

 the Moon visits the northern and southern sides of 

 the Earth. Dr. Hecker has taken out the averages 

 separately for these " Lunar Summers '" and " Lunar 

 ^^'inters,"" Figure 6 gives the doings of the pendu- 

 lums when the Moon was far North, the full line 



representing what is to be expected 



steel 



(I o/o 



FiGU 



rigid in the Northerh' 



Earth : and similarh- Figure 7 gives the same facts 

 for the days when the Moon is in Declination far 

 South. In both figures both the acccordances and the 

 discordances between theory and observation are 

 remarkable. The double loop is in good agreement ; 

 the hours against each point show perfect harmonv 

 between theorv and observation : especialh' when it 

 is noted that twelve o'clock 

 comes on the larger sweep in 

 one case, and on the smaller 

 in the other. There can be 

 assuredly no question that the 

 pendulums are recording the 

 phenomenon which was sought 

 for. The chief discrepancy is 

 in the fact that observed 

 curves are distinctly dwarfed 

 in the direction North and 

 South. This has received 

 close attention at Dr. Hecker's 

 hands, and all he can sa\- is 

 that we must assume that the 

 continent of Europe is less 

 and Southerly direction than 

 in the Easterly and Westerly direction. 



In conclusion, there will probabl\- arise in the 

 mind the question of Earthquakes. Will this 

 deformation of the Earth's surface tend to produce 

 or perhaps even to account for Earthquakes ? At 

 present it is too early to venture on any answer to 

 the question. We must first make up our minds 

 whether Earthquakes are rendered more frequent h\ 

 greater or by less rigidity. At flrst sight one would 

 say that the more rigid the structure is the less it 

 will quake ; but then, again, the more nearly it 

 conforms to the shape that the forces on it make for, 

 the less reason is there for it to quake, the less stress 

 is it under. It is impossible to say from general 

 considerations which of these facts will prevail. 

 Earthquakes are now being largely attributed to the 

 readjustment of the shape of the Earth in response 

 to changed circumstances of rotation. The Earth's 

 figure is held to be somewhat out of date ; she is too 

 flat at the Poles and possibh- not quite so perfectly 

 circular at the Equator as she should be. But in 

 such arguments the changes of circumstances are 

 extended over millions of j'ears, while these Tides in 

 the crust are recurring twice each day. For the 

 present we must be content to leave their possible 

 connection with Earthquakes on the knees of the 

 gods. 



