358 Royal Society. 



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 springtides 8'74; mean 17'649. The 

 mean high-water neap tides 22 '65 6, low-water 11 '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 

 was rendered tedious by the abruptness of the ascent. Having com- 

 pleted these observations, it occurred to the Astronomer Royal that 

 the instruments employed in the survey might be used as a means of 

 verifying the correctness of the horizontal point of the mural circles. 

 The coincidence of the horizontal wires of the two instruments was 

 found to be so nearly perfect, as to agree within a few hundredths 

 of a second. From Greenwich the levelling was continued on the 

 opposite side of the river to different places where tide-registers had 

 been kept. By the kindness of Mr. Lubbock, the author was fur- 

 nished with the results of twenty-six years' observations on the tides 

 at the London Docks ; from which it appears, that the height of mean 

 high- water mark there, above that at Sheerness, is 2*24 feet, and the 

 height of spring tide high-water mark 2'03, and of neap tides 2*35. 

 The Trinity mark on the western side of Old London Bridge is 

 2' 16 feet below the north standard mark at Sheerness, and 1'9 foot 

 above the mean spring tide high-water mark at Sheerness. 



The author concludes by giving a long catalogue of standard marks 

 and other points of reference between Sheerness and London Bridge, 

 the north standard at Sheerness being taken as the zero point. 



In the course of his observations he found reason to believe that 

 the tremulous appearance of the air which has been termed mirage* 



is 



