C 134- ] 



XXI. Remarks on some Tide Observations, published in the 

 Transactions of the British Association. By RICHARD 

 THOMAS, Civil Engineer*. 



IN the Transactions of the British Association for the year 

 1838, is an account of a level line measured from the 

 Bristol Channel to the English Channel, from which the writer 

 draws the conclusion, " that the mean tide must be taken as 

 the level of the sea" and that this mean tide is the same, or 

 nearly the same elevation at all places, whatever may be the 

 differences of rise and fall. 



About three years ago the British Association published 

 the results of certain tide observations made at Liverpool, 

 which induced me to write the following letter on the subject, 

 which was inserted in the ' Mining Journal,' and " Falmouth 

 Packet" Newspapers. 



" In the supplement to the ' Mining Journal,' No. 23, 

 there appears to be published under the sanction of the British 

 Association, ' that there is one invariable mean height common 

 to neap and spring tides, the HALF TIDE MARK, a point from 

 which engineers, geologists, and navigators will henceforward 

 commence their calculations and adjust their standards of 

 comparison.' 



" I have reason to believe that however accurate the conclu- 

 sion is with regard to the tides at Liverpool, where the obser- 

 vations were made, it is not correct as to its general applica- 

 tion, and I mean to show that the tides generally have not the 

 same elevation of half- tide mark, as applies to any particular 

 locality, nor is the average half-tide mark, nor low-water 

 mark, nor high-water mark at one part of the coast to be de- 

 pended on as level with the corresponding tide marks on other 

 parts. More than twenty years ago I had occasion to attend 

 particularly to the tides at Falmouth, and the result of my 

 observations showed differences as much as two feet and a 

 half in elevation on half-tide marks. The rise and fall of 

 ordinary spring tides there is about 17 feet, and of ordinary 

 neaps little more than seven feet, but the several rounds of tides 

 differ as to the mean elevation of the sea ; the low- and high- 

 water marks for the same difference of rise and fall being at 

 greater elevations than others. 



" At King- Road near Bristol, I observed the tides in the 

 year 1815, and found that the difference of elevations of some 

 half-tide marks amounted to about four feet. 



" It may be possible, and I think it probable, from my ob- 

 servations, that these differences of half-tide levels, or rather 



* Communicated by the Author. 



