8 EIGHTH RErORT — 1838. 



Thus the sea ;it Portishead is at high water 13*6 feet higher and 

 at low water 12-14 feet lower than at Axmouth. ' And if we take 

 the extreme tides whicli occurred during the observations, the 

 differences are still greater ; for the greatest range of tides at 

 Axmouth was 10*8 feet, and at Portishead 41*1 feet. And the 

 difference of the halves of this is 15-1 feet, which is greater by. 

 2-23 feet than the difference of the ranges just employed. Also 

 these elevations of the ocean are nearly contemporaneous ; for 

 the high water at Axmouth occurs (at a mean) forty-four minutes 

 earlier than at Bristol, and at Portishead two minutes and three 

 quarters later than at Bristol. 



We have in these results a very strong indication that the 

 mean tide is what we must take as the level of the sea, for it 

 would be difficult to believe that the level of the sea is fourteen 

 feet higher at Portishead than at Axmouth, or sixteen feet lower, 

 which are the consequences of taking for the level high water 

 or low water at spring tides*. 



We may add, that at another of our stations, the Wick rocks, 

 by a month's observations in November, 1837, the mean level 

 was 73*11, or 3*8 inches higher than at Portishead. Perhaps a 

 portion of this difference may be due to the inevitable errors of 

 the operations. The range of the tides at this place is nearly 

 the same as at Portishead, and the time of high water (at the 

 mean) about thirty-seven minutes earlier than Bristol, and there- 

 fore about seven minutes later than Axmouth. 



15. The general result to which we are led is that the mean tide 

 must he taken as the level of the sea. This result had already 

 been arrived at by various persons. Capt. Denhani had asserted 

 it as the consequence of his observations at Liverpool ; and Mr. 

 Walker had been led to the same conclusion by the tides of Ply- 

 mouth. I had also pointed it out as the result of the Plymouth 

 observations in the Philosophical Transactions for 1837- 



] 6. But these conclusions were supported only by observations 

 made at a single place ; namely, by its appearing that the height 

 of mean tide was nearly constant, (varying at most only a few 

 inches) while both high and low water varied by many feet. 

 And so far as I have yet seen the evidence, it seems probable that 

 though the change of the level of mean tide during a fortnight 

 be small, there really is some regular change of this level tide, 

 produced by the effects of the moon and sun. But the small- 

 ness of the "changes of this level, as it is now announced, rests 

 upon quite different evidence, and appears to indicate a perma- 



• In Plate II. I have lepfcsenled the relative range of the tides at these 

 places as observed. 



