REPORT — 1871. 



85. It vaU. be seen, on comparing the results contained in the previous 

 lleport with the above, that the chief tides (the lunar and solar semidiurnal) 

 are now more retarded by about 4° than during the j-^ears previously analyzed. 

 The calculated heights in the comparison should therefore more nearly repre- 

 sent the heights about eight minutes after the hours assigned to them. An 

 examination of the differences will show this to be the case. A fresh calcu- 

 lation and due aUowauce made for atmospheric pressure would doubtless very 

 considerably reduce the discrepancies. 



86. The gradual increase in the height of the mean level of the water (A„), 

 probably arising from the filling in of the bed of the river and consequent 

 increase of friction, will account for some portion of this increased retarda- 

 tion. There was a very violent rise in the mean level for the year 1868-69, 

 amounting to four tenths of a foot ; it, however, in the following year had 

 again subsided to about its anticipated height. The uncertainty in the mean 

 level of the water is an element which must at times seriously affect the 

 differences between calciilated and recorded heights in any method of com- 

 putation of heights from ajixed datum. With respect to these changes now 

 taking place in Liverpool Bay, the following extract contains the substance 

 of the Marine Surveyor's report, dated October 2nd, 1S71, and confirms the 

 results determined by the i)receding reductions : — 



" The result of the survey of the channels of the river for the current year 

 shows that the changes rendered necessary in the arrangements of the light- 

 ing and buoyage are more important in their immediate effect on the course 

 of navigation than any which have occurred .for some years. The present 

 Queen Channel was opened in 1854, but was not buoyed for navigation pur- 

 poses until two years afterwards. Since that time the process of advance 

 from southward to northward of the Great Burbo Bank had been very 

 gradual for the first ten years, but more rapid recently, so that the advance 

 had extended to about half a mile. At the same time the North Channel 

 had widened in the same proportion, and there was no appreciable narrowing 

 of the channel in that direction. On the north side, however, during the 

 last four years, the changes had been more rapid, and the buoys had been 

 altered twice within that period. It was now necessary that the Bell Beacon 

 should be removed northward one third of a mile, and also that the Formby 

 light-ship should be removed one third of a mile westward. Thus the Bell 

 Beacon buoy would be brought into a direct line with the Crosby light-ship. 

 The bar was in a satisfactory state, and the whole of the channels were in 

 as safe a condition as they had been for many years, being deeper as well as 

 more straight and not narrower than formerly." 



87. It is very much to be regretted that the authorities at Liverpool 

 have chosen the George's landing-stage for a tide-float, affected as it must 

 be (sometimes to a considerable extent) by the ever-varying weight it has to 

 bear. This will affect the whole of the tide-components evaluated, but more 

 especially the solar components, and will account for the different values of 

 the solar semidiurnal tide, which, judging from the corresponding lunar 

 component, should agree within much narrower limits. It is therefore 

 thought that, should it be determined to again discuss the Liverpool tides, 

 it will be better to take the tide-curves as self-registered at Helbre Island 

 at the mouth of the Dee, in preference to those of George's Pier. The Helbre 

 Island tide-curves it is considered will give much superior results. 



88. Through the kindness of the United States' Coast Survey Office, two 

 years' tide observations, taken at Fort Point, San Francisco Bay, California, 

 being a continuation of the observations already analyzed (§ 66), have been 



