EARTH TIDES. 



K\ 



HAKDCASTLK. F.R.A.S. 



Those who take up the study of some subject after 

 their school-days are over, usually have to dispense 

 with a teacher, and, in the case of astronomy, such 

 people have recourse to books of a general descriptive 

 nature, which carr\- them superficialh' over the whole 

 range of the subject. Many such books are 

 unquestionably stimulating, but they cannot possibly 

 be otherwise than shallow, and it will be well to 

 provide an antidote bv picking up one thread and 

 following it as far as possible, going as deep as the 

 equipment, both mental and instrumental, will permit. 

 In this way it is possible to penetrate into the real 

 spirit of science, or rather to allow 

 the spirit of research to penetrate 

 into one"s soul. For instance, the 

 student will thus realise the sort of 

 difficulties that have to be overcome 

 while, bv the 



7b f^Don . » 



mere perusal of 



the ordinary 



text - book, 



there is left on the mind the 



impression that there are no 



difficulties, because of course, ever\- 



thing is purposely made easy and 



clear. The tides are an excellent 



example of a thread which will 



repay time and trouble. Any 



almanac will give the times of high water, and in 



those columns will be found faithfully reflected ever\- 



vagarv of the Moon's movements. The disentangling 



of them requires no abstruse mathematics : interest 



and perseverance are all that is needed. 



In the history, too, of astronomy, the tides have 

 played an important part. Xewton"s explanation of 

 them forms one of the most brilliant sections of his 

 Principia. As clearly as the mythical apple thev 

 demonstrate that the forces that we know on earth 

 are not limited to the earth : it ma\- be confidentlv 



asserted that those forces bear swav throughout the 

 solar system, and, as we mav now add. in the 

 systems of the double stars. Perhaps some dav we 

 may show that gravitation works even across stellar 

 distances, but it is well to remember that at present 

 we have no evidence of this. We can. however. 

 endorse the poet's generalisation : — 



That ever>- law that moulds a tear 

 And bids it trickle from its source. 



That law has formed the earth a sphere. 

 And guides the planet in its course. 



In the history of the tides the most fascinating 

 chapter will undoubtedly be that which tells of the 

 researches of Sir George Darwin. He has shewn us 

 that ever\- high water that washes our coasts is a 

 manuscript note from the hand of the Moon herself 

 to tell us of her life-history — an autobiographical 



note — if we would but read, if we could but under- 

 stand. We may give a literal meaning now to 

 Addison's words : — 



Soon as the evening shades prevail 

 The moon takes up the wondrous tale. 

 And nightly to the listening earth 

 Repeats the story of her birth. 



And we must now approach the latest chapter in 

 the history of the tides. It announces the regular 

 upheaval of the solid crust of the earth twice ever\- 

 day through a range of perhaps six inches. 



That something of the kind must be taking place 

 has long been suspected from 

 general inferences about the forces 

 exerted bv the Sun and Moon, and 

 the known properties of the materials 

 of our Earth, and it lias been 

 cvnicallv pointed out that no 

 moment is fraught with greater 

 -O danger to science than when some 

 theoretical prediction is awaiting 

 tlie confirmation of actual obser- 

 vation. No one, however, who 

 reads the paper by Dr. Hecker in 

 the Publications of the Prussian 

 Geodetical Institute will suspect him 

 of self-deception in thinking that he 

 has obtained direct evidence of the 

 existence of tides in the solid crust of the Earth. 



In order to appreciate the magnitude of the 

 triumph it will be necessary- to realise the nature of 

 the forces to be investigated and some of the 

 difficulties to be contended with. And this is, after 

 all, true in every piece of scientific research ; the real 

 interest lies not in the results but in the methods. 

 It is perhaps one of the greatest disservices that 

 writers who popularise such subjects habitually 

 render, both to the public and to the cause of science, 

 that thev represent the final outcome of some 

 investigation as the one and onl\- point which 

 matters. It is probabh', on the contrary, the one 

 thing which the future will repudiate. 



What is a tide ? The answer that first suggests 

 itself is the regular ebb and flow of the water over 

 the sands or beach. This is what strikes the eye, 

 and it requires but little thought to see that this flow 

 of the water is due to the rise and fall of the whole 

 sea level in that neighbourhood. The sea. in fact, 

 becomes deeper and shallower twice each day. The 

 usual diagram to represent the tides is shown in 

 Figure 1. 



For simplicit\- the action of the Sun is here 

 ignored ; the greater depth of the water at A and B 

 represents the two tides that the Moon causes. It 

 is of course obvious that the diagram assumes that 

 the whole Earth is covered with water, and also 



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