Mr. Nixon on the Heights of the Hills of Dent, Sfc. Yorkshire. 189 



fresh water arid marine shells intermixed. This deposit* may 

 naturally be allowed to mark the highest tide of those days. And 

 if it is found that the present level of high water is below the more 

 ancient one in the valleys, it is proved at once that the ocean has 

 receded at that particular district ; but if the levels are found to 

 be nearly the same, it will be proved that the sea has been ex- 

 cluded, and thus set the question completely at rest : at present 

 I lean towards the latter opinion, from circumstances in some 

 degree similar on the Kentish coast near Hythe. The sea was 

 known to have come much nearer the hills formerly, than it now 

 does ; but the level of the Military Canal proves that the level 

 of high water has not decreased, and that the change has taken 

 place by the formation, by the ocean, of natural barriers of 

 shingle, over which it very rarely flows, and even then in such 

 small quantities as to be harmless ; whilst the lower parts of 

 Romney Marsh are liable to inundations to a great extent when- 

 ever a breach is made in Dymchurch, or the other walls : 

 thus adding proof that the sea has not decreased on this coast, 

 but is kept out by natural and artificial boundaries. 



It would add to the proofs, if the mean of the low water 

 level was also carried on, and the bottom of the ancient sandy 

 deposit bored to, and measured. 



Sittingbourne, Feb. 4, 1828. , J. , 



XXXI. On the Measurement by Trigonometry of the Heights of 



the principal Hills in the Vicinity of Dent, Hawes, and Sed- 



bergh, in Yorkshire. By John Nixon, Esq. 

 [Concluded from p. 95.] 

 r j^HE vertical angles were measured by the horizon-sector, 

 ■"* described in the Philosophical Magazine, vol. lix. p. 130. 



At Noughtberry Hill the instrument stood upon a large 

 square board screwed firmly to the tripod of the larger theo- 

 dolite, and at Pen-y-gent it was placed upon the well-built 

 wall crossing the summit of the hill ; but at the other stations 

 a flag firmly fixed upon the signal-tower reduced to an alti- 

 tude of about four feet, formed the rude yet incomparable 

 stand. 



Formerly the minute error of adjustment of the levels, which 

 was found to vary with the temperature, was ascertained when 

 on the point of setting out for the station, and a register of the 

 thermometer kept contemporary with the observations. To 

 this plan two objections existed : the error of adjustment might 



* 1 mean the upper surface of the one Mr. Robberds mentions, in speak- 

 ing of the soil of the meadows and marshes ; as " the lowest bed of their 

 soil is sand in which marine shells are found." 



have 



