PHYSICAL ASTRONOMY. 563 



The Tides of the Coast of Ireland have been examined with great 

 care by Mr. Airy. Numerous and careful observations were made 

 with a view, in the first instance, of determining what was to be re- 

 garded as " the Level of the Sea ;" but the results were discussed so as 

 to bring into view the laws and progress, on the Irish coast, of the 

 various inequalities of the Tides mentioned in Chap. iv. Sect. 9 of this 

 Book. 



I may notice as one of the curious results of the Tide Observations 

 of 1836, that it appeared to me, from a comparison of the Observa- 

 tions, that there must be a point in the German Ocean, about midway 

 between Lowestoft on the English coast, and the Brill on the Dutch 

 coast, where the tide would vanish : and this was ascertained to be 

 the case by observation ; the observations being made by Captain 

 Hewett, then employed in a survey of that sea. 



Cotidal Lines supply, as I conceive, a good and simple method of 

 representing the progress and connection of littoral tides. But to 

 draw cotidal lines across oceans, is a very precarious mode of represent- 

 ing the facts, except we had much more knowledge on the subject 

 than we at present possess. In the Phil. Trans, for 1848, I have re- 

 sumed the subject of the Tides of the Pacific; and I have there 

 expressed my opinion, that while the littoral tides are produced by 

 progressive waves, the oceanic tides are more of the nature of station- 

 ary undulations. 



But many points of this kind might be decided, and our knowledge 

 on this subject might be brought to a condition of completeness, if a 

 ship or ships were sent expressly to follow the phenomena of the Tides 

 from point to point, as the observations themselves might suggest a 

 course. Till this is done, our knowledge cannot be completed. De- 

 tached and casual observations, made aliud agendo, can never carry us 

 much beyond the point where we at present are. 



Double Stars. 



Sir John Herschel's work, referred to in the History (2d Ed.) as 

 then about to appear, was published in 1847. 10 In this work, besides 

 a vast amount of valuable observations and reasonings on other subjects 



10 Results of Astronomical Observations made during the years 1834, 5, 6, 7, 8, at 

 the Cape of Good Hope, being the completion of a Telescopic Survey of the whole Sur- 

 face of the visibk Heavens commenced in 1825. 



