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imperfections in the levelling instruments hitherto used, he bestowed 

 great pains in improving the construction of those employed in the 

 present survey ; endeavouring to combine the properties of great 

 steadiness and accuracy of motions in azimuth, with increased deli- 

 cacy in the level, and permanence in the general position of the 

 whole apparatus j and also to increase the power of the telescope. 

 The author then enters into a full description of the improved in- 

 struments which he employed, accompanied by drawings. 



As soon as he was furnished with the proper means of observing, 

 he commenced his operations at Sheerness in the month of March. 

 The principal object of his commission being to ascertain the heights 

 of different places above the level of the sea, it became necessary, 

 in the first place, to estimate the heights of the tide; and accord- 

 ingly, having obtained permission from the Admiralty to erect a tide 

 gauge at the Dock Yard at Sheerness, he selected a corner of the 

 boat basin as the most eligible spot for this purpose : having accom- 

 plished this object, he next directed his attention to the establish- 

 ment of a standard mark, from whence, as from a zero point, the 

 levellings might be reckoned. Considerable difficulty was met with 

 in fixing upon a spot in every respect adapted to this object ; for 

 all the buildings in the immediate vicinity of the tide gauge appeared 

 to be deficient in the security of their foundation. He at length se- 

 lected a large block of granite in the southern pier of the entrance 

 to the boat basin. He then caused a block of gun metal, cast for 

 the purpose, two inches and a half square and eight inches long, to 

 be sunk in the centre of the granite, about an inch below the sur- 

 face, thereby allowing a brass box and cover to be placed over the 

 standard to protect it from injury. In order that there might be a 

 sufficient number of checks to the permanence of this standard 

 mark, the author caused others to be placed in the yard ; namely, 

 one near the southern extremity on the wall of the Dock Yard, one 

 at the eastern side of the great basin, and one in a large block of 

 stone resting on the brick-work of the navy wall. As a further 

 means of future verification of this standard level, he had a very 

 large block of granite placed on a slight eminence, two miles and a 

 half to the southward of the Dock Yard, on which there formerly 

 stood the old castle of Queenborough. One of the brass standards 

 being let into the granite, the place was covered over, but marked 

 by a small mound of earth near it, so that it may easily be referred 

 to whenever it may be thought requisite. 



From a series of observations made at Sheerness in the years 1827, 

 1828, and 1829, it is found that the mean high-water spring tides 

 was 26-355 feet, low-water spring tides 8*74 ; mean 17 649. The 

 mean high-water neap tides 22 656, low-water 1 1 -336 ; mean 16 993. 

 The mean of the whole period being 17"27. 



The author then states the results of the successive levellings he 

 took from Sheerness along the course of the river to London Bridge. 

 On his arrival at Greenwich Hospital, he commenced a set of branch 

 levels from thence to the Royal Observatory, for the purpose of de- 

 termining its height above the level of the sea, — an operation which 



