Mr. J. Nixon on the Tides in the Bay of Morecambe. 269 



sinking of the heavy instrument within the sand, which re- 

 quired that both levels should be taken as nearly at the same 

 time as possible*. The total fall appeared to be 18 feet 

 6 inches, but a solitary limestone, considered as the standard 

 height for extreme neaps, was situated 1 foot 9 inches below 

 the level of our mark, which would reduce the total height of 

 the tide by double (?) that quantity, or to 15 feet. 



July 28, 1832, the day previous to the highest spring tide 

 (the weather being perfectly calm), the fall from the level of 

 high water (at ll h 32 ra a.m.) to our stake near the shore mea- 

 sured 6 feet 5 inches, but that from the stake by the lake 

 down to the level of the latter at low water (at 5 h 57 m p.m.) 

 did not exceed 4 feet 7 inches, although a continuous fall was 

 perceptible nearly up to the time quoted f. The total height 

 of the tide will therefore be 29 feet 3 inches. On this occa- 

 sion, as the theodolite was completely adjusted, the difference 

 of level between the two stakes driven down at neaps was 

 carefully remeasured, and made to be 18 feet 3 inches, or only 

 3 inches less than previously. 



In strictness the rise as well as the fall of a tide should be 

 obtained, and the mean taken for its correct height. 



Measurement by the Dip of the Height of the Tides. 

 No ray of light originating at any point (P) on the (sphe- 

 rical) surface of the sea 

 (PL) could pass below 

 the level of the hori- 

 zontal plane PD (which 

 is perpendicular to the 

 vertical PC) without 

 being absorbed in the 

 sea. At the point of 

 intersection (D) of this 

 plane with any other 

 vertical, as DC, the an- 

 gle HDP, or depres- 

 sion of P below the 

 horizontal plane or line 

 DH, is termed the true 

 dip, and is equal to the 

 arc contained between 

 the two verticals PC and DC, or the angle PCD. 



* As an additional precaution one staff was read off before as well as 

 after noting the other, and the mean of both readings registered. 



f The fishermen hold that there is a difference of half an hour between 

 the duration of ebb and flow in Hevsham lake. 



