Mr. J. Nixon on the Tides in the Bay of Morecambe. 271 



tolerably successful, would probably have been completely so 

 had the base of Walney lighthouse (the object selected for re- 

 ference) been observed in lieu of its ill-defined conical top. 

 At Hest-bank wall, where the measurements were made by 

 the sector, the height, though scarcely more than requisite 

 to obtain at low water an unobstructed view of the open sea, 

 throws its horizon to a distance of 9 to 1 1 miles, and thus 

 renders it necessary to have the dip true to 3" or 4" in order 

 to insure to the calculated height of the eye the accuracy of 

 one foot. The bank is steep, and extends to the south-west 

 and north-east. At high water the station may be 200 yards 

 distant from the sea in the direction observed, (a little left of 

 the lighthouse, which lies to the west by south,) but at low 

 water sands, intersected by channels and pools of water, in- 

 tervene for several miles. 



Generally each day's observations are arranged below, in 

 an order to exhibit, — 1st, The variation at low water of the 

 depression of the lighthouse from its amount at high water; 

 2nd, the refraction at high water, or difference between the 

 observed dip and that calculated from the measured height of 

 the eye ; 3rd, the height of the eye at low water, computed 

 from the observed dip corrected by a refraction differing from 

 that obtained at high water by the corresponding variation in 

 the depression of the lighthouse ; 4th, the dip answering to 

 the height of the eye at low water, derived from a table, 

 founded on the measurements at Heysham ; 5th, the conse- 

 quent error of the estimated refraction and height of the eye. 



September 14th, 1829. 



(At h 5 ra P.M. Black Comb, 1 42 ' 21" elev • refr U 

 126,325 feet distant/ 4 ^ Zi eleV * ' retr ' ™ 



10 17 a.m. Lighthouse topi , j 



9,863 feet distant/ d * 6 Uepr ' \ 5' 46" 

 K 4Q — J 



5 p.m 5 49 



* 2 ° 5 34i - 



6 10 5 31£ 



} 



Variation 13 



of the hang of the tide. Granting even a difference of level in the sea 

 at high water at the two localities, we should then form a false estimate 

 of the refraction, yet as the calculated dip at low water would he nearly 

 equally in defect or excess with that at high water, their difference, or 

 height of the tide, would not be materially affected. 



