REPORT ON A LEVEL LINE. 7 



observations were made shnidtaneously at Axmouth and at Por- 

 tishead, from July 16 to 30, 1838 ; the result of which was that 

 the level at Portishead is nine inches higher than that at Axmouth. 



13. The difference between the result of the first and second set 

 of tide observations at Axmouth (1"29 feet in the mean level,) 

 was such as to require examination. It appeared possible that 

 this difference might arise from some of the inequalities which 

 affect the tide, and depend upon the time of year ; one set of 

 observations having been made in January 1838, and the other 

 set in July. I therefore requested Mr. Bunt to examine the Ply- 

 mouth observations of high and low water for the same period 

 (with which observations I was supplied by the Admiralty). The 

 result of this examination was that the mean sea levelat Plymouth 

 was only one fiftieth of a foot higher in January than in July 

 last : and it therefore appears certain that no annual inequality 

 of the tides is the cause of the difference. T am led to ascribe 

 it to the circumstance, that in the observations of January, the 

 low water at Axmouth was taken within the bar at tiie mouth of 

 the river. In Julj'^, the low water, within this bar, was certainly 

 higher by a foot or two than it was on the outside ; and though the 

 bar had altered its position in the intermediate time, I have little 

 doubt that it was in such a condition in January as to vitiate 

 the observations of low water. The observations of low water 

 made in July last, simultaneously with those at Portishead, were 

 made entirely outside the bar. 



14. Taking the simultaneous observations made at Axmouth 

 and Portishead in July, 1S38, as the most free from obvious objec- 

 tions, we obtain the following results respecting the comparative 

 level of the sea at the two extremities of our line. The measures 

 of level were all referred to a certain zero point, assumed 100 

 feet below the point where the operations began. The level of 

 the mean tide above this zero was 



at Axmouth . . . 71*96 

 at Portishead . . . 72-69 



difference . . '73 foot; 

 or something less than nine inches. But the range of spring tides 

 at Axmouth was 5 feet above and below this level ; at Portis- 

 head 17"87 feet above and below. Hence we have for the relative 

 levels of high and low water at spring tides 



High Water. Low Water. 



at Axmouth . . 76-96 . . . 66-96 

 at Portishead . . 90*56 . . . 54-82 



differences . . 13-60 . . .12-14 



