178 REPORT — 1875. 



between neaps and springs, are sliown on a diagram plotted from observations 

 made continuously for a fortnight. In this diagram (Plate VII. fig. 1) the actual 

 heights of high and low water of each tide are plotted above or below Ordnance 

 datum, and then two equalizing lines, drawn as a mean of the observations, 

 serve to show what would be the higli or low water for any given range of 

 tide from 15 up to 46 feet. 



The same diagram shows also how far the mean half-tide level at the 

 mouth of the Avon agrees with the theoretical mean sea-level, as adopted for 

 the Ordnance datum. The half height of each range of tide, taken in the 

 above-mentioned observations, is plotted, and a mean equalizing line drawn 

 between them. The result shows that at lowest neaps the half-tide level is 

 bout 3| inches below, and at highest springs rises to about 1 foot 8 inches 

 above Ordnance datum. Other observations made at the moiith of the Avon 

 tend to confii-m the conclusion that, so far as regards our local tides, the 

 mean half -tide is not a fixed level, and that it is above the Ordnance datum. 

 This may point to the probability that the mouth of the Avon is somewhat 

 within the influence of the surface fall of the lowest part of the Severn, and 

 above the true level of ocean low water. 



In connexion with the subject of meau sea-level, it may not be uninter- 

 esting to notice that, at the time of the last Meeting of the British Association 

 at Bristol, in 1836, the question was much under general disciTssion, and it 

 ■was resolved that a series of levels should be taken between the Severn at 

 Portishead and Axmouth on the English Channel. These were undertaken 

 and carried out by the late Mr. T. G. Bunt in 1837, and were conducted 

 ■with an amount of care and skiU to secure accuracy which has seldom been 

 exceeded. In connexion with the stations levelled to at either end of this 

 line, a series of simultaneous tidal observations were made, by which it was 

 found that the sea at Portishead rose at high water 13 feet 7 inches higher, 

 and fell at low water 12 feet 2 inches lower than at Axmouth, the total 

 difference in the ranges of the same tide at the two places being as much as 

 25 feet 9 inches. This is a very striking illustration of the effect of the 

 momentum of the incoming tide-wave heaping up the water in this funnel- 

 shaped estuary. 



On looking at the map it will be seen that the course of the Avon lies at 

 about right angles to that of the Severn ; and its tide may be considered to 

 be generated, as it were, by the passing tide of the Severn, rather than directly 

 due to the momentum of the original tide-wave. As the flood-tide rises in 

 Kingroad it pours into the Avon, and a current is established in the latter 

 river which soon obtains a momentum of its own. The effect of this is very 

 plainly seen, and serves to illustrate the same phenomena of engorgement 

 •which takes place on a larger scale in the Bristol Channel, for the tide rises 

 to a higher level in the Avon the further we go up the river. Taking the 

 flood of an equinoctial spring-tide, we find that at the mouth of the Avon 

 high water rises to 24 feet 10 inches, at Sea Mills to 25 feet 3 inches, at 

 Cumberland Basin to 25 feet 5 inches, at Netham Dam to 25 feet 9 inches, 

 and finds its summit at a point about six miles above Cumberland Basin, 

 where it rises to 26 feet 4 inches, all above Ordnance datum. Here the 

 momentum, as we have seen in the case of the Severn, becomes spent, and 

 the rest of the tide has a reversed slope up the freshwater river to Hanham, 

 where its level is only 26 feet above datum. 



On the longitudinal section of the Avon exhibited, the points above men- 

 tioned are shown, as also some cross sections of the river, the slope of the 



