THE TIDES 539 



Attention has been called to these local details for the purpose of 

 showing how varied the tidal phenomena may be within a compara- 

 tively limited region, and that detailed cotidal maps can be constructed 

 with accuracy only after tidal observations have been made at a great 

 number of points along the shores and upon the islands included in 

 the area to be represented. The gauging of the tide away from land 

 has seldom been undertaken; but there are indications that in the fu- 

 ture such work will be carried out on an extensive scale. 



Concluding Eemaeks 



From the cotidal maps of the world, the reader can draw his own 

 conclusions concerning the resemblance between the tides as they must 

 approximately occur in nature and the hypothetical tides progressing 

 westerly around the globe according to the popular conception of to- 

 day and which, as we have seen, existed at the time when the Ptolemaic 

 System held sway over the astronomy of medieval times. He will find 

 no indications of tide waves having crests coincident with meridians 

 and progressing westward at the rate of fifteen degrees of longitude 

 per lunar hour. He will see that even a general westerly progression 

 of the tide around the globe has no existence in fact. 



The dictum of Aristotle, Pliny, Newton and others, that large tides 

 are found in large seas, can be tested by means of these charts. The 

 truth of this is manifest when the tides of the Mediterranean Sea are 

 compared with those of the North Atlantic Ocean, but no such rule 

 applies when the tides of the North Atlantic Ocean are compared with 

 those of the Pacific Ocean and especially with those of the South Pacific 

 Ocean. 



The reader may ask. How does it happen that even rude approxima- 

 tions to the tides in nature have been so slow in their development? 

 The answer is. Definite or applicable theories concerning the causes of 

 existing tides were impossible before the depths of the oceans had be- 

 come generally known. And so, before a few decades ago, tidal theories; 

 were developed with reference to certain assumed or hypothetical cases'. 

 This remark is no disparagement of the labors of such men as Newton, 

 Laplace and Airy, and which compel the admiration of all persons in- 

 terested in tidal matters. In fact, the comparative simplicity of the 

 hypotheses upon which their theories were founded, placed tidal work 

 squarely upon a scientific basis and enabled these intellectual giants to 

 press their investigations a long way towards completion. 



As to the construction of cotidal lines for the various oceans, it may 

 be said that this implies, not only a rational approximate theory, but 

 also a multitude of carefully-made tidal observations ; such observations 

 are, even to-day, either wanting or defective in many portions of the 

 globe despite the efforts of individuals, expeditions, learned societies, 

 institutions and governments. 



