163 REPORT — 1875. 



tion, it may be confidently anticipated that the self-recording of the tides 

 may be again soon resumed. 



By the courtesy of the present Marine Surveyor to the Mersey Docks and 

 Harbour Board, Staff-Commander Graham H. Hills, E.N., the writer has 

 had an opportunity of inspecting a number of the resultant diagrams of the 

 tidal curves. 



After careful examination of many tides, the writer confesses himself quite 

 unable to agree with Capt. Denham in his definition that " the half-tide level 

 is an oscillating point or mean centre which every six hours is common to 

 aU tides, whether spring or neap," or with the assertion "that the oscilla- 

 tion of each tide, whether spring or neap, passed this line at three hours 

 before and three hours after every high- water time." 



Some difterence of opinion seems to exist as to the interpretation of the 

 term " mean or half-tide level." The method by which this level appears 

 commonly if not generally to be arrived at is by taking the meau of a 

 number of levels of high water for a mean high water, then of a nearly 

 similar number of low-water ones to form a mean low-water level, the 

 difference between these two means giving a mean tidal range, the half of 

 which amount, being reckoned upwards from the mean low-water level, 

 gives an absolute level called the mean or half-tide level — in fact the 

 mean half-range of the tide. 



"Whether this is exactly what Capt. Denham meant by his "oscillating 

 point or mean centre," is uncertain. But it is without doubt that this half- 

 range varies considerably, not only with successive tides, but even over 

 lengthy periods, such as a year in duration, or even a duration of years. 



The writer has been enabled, by the courtesy of the Marine Surveyor, to 

 place upon the diagram hereto appended (Plate IV.) the annual means at 

 Liverpool for high water, for low water, for tidal range, and for the haK-tide 

 level for the twenty years extending from 1854 to 1873 — each year being 

 represented by a vertical line with its date upon it, and the position of its 

 mean of high water, low water, and half-tide level indicated upon it to a scale 

 of a quarter of an inch to one foot. The irregular line at the top of the diagram 

 is formed by the junction of the annual means of high water, the mean high 

 water for the twenty years being indicated by a horizontal line. The low- 

 water means are similarly treated at the bottom of the diagram ; while in 

 the middle of the sheet the half-tide levels are described in a like way. 



The datum to which they are reduced is the sUl of the Old Dock at Liver- 

 pool, a datum much in use in that neighbourhood. 



A very brief inspection of the diagram will show that there is considerable 

 variation in each of the means between different years, amounting, in more 

 than one case, to nearly twelve inches. And if so with annual means, how 

 much greater the irregularity in shorter periods ! 



If, therefore, by " half-tide, level " Capt. Denham means the level of half- 

 tide range, his idea of " an oscillating point common to all tides " cannot 

 hold good, seeing that the result of the twenty years' observations shows 

 considerable differences to have existed in the "half-tide level" between the 

 several years, in some cases nearly nine inches. 



It may, however, be urged that some of the variations occurring in the 

 course of this lengthened period are caused by the altered form of the course 

 of the tidal channel of the River Mersey near to its mouth, to its being gra- 

 dually narrowed and made more direct by each successive prolongation of the 

 liver-wall of the Liverpool Docks. 



While admitting that something may be due to this cause (a matter not 



