32 



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 w T iththe 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 caused, not so much by evaporation as by the direct effect of the 

 solar rays : for he remarked, that when there was a succession of 

 clouds passing over the sun, the tremor was very great at those times 

 when the sun shone ; but the moment the sun was obscured over the 

 whole space between the instrument and the object viewed, theair was 

 perfectly tranquil. 



March 10. 



HIS ROYAL HIGHNESS THE DUKE OF SUSSEX, 

 President, in the Chair. 



Alexander Caldcleugh, Esq. and John Carnac Morris, Esq. were 

 elected Fellows of the Society. 



The following Presents were received, and thanks ordered for 

 them :— 



An Engraved Portrait of Davies Gilbert, Esq. V.P.R.S. Engraved 

 by Cousins from a painting by Howard. — Presented by John Guil- 

 lemard, Esq. F.R.S. 



Notices of the Proceedings of the Zoological Society. (Jan. 25 to 

 Feb. 8, 1831.) S\o. — The Society. 



It being stated to the Meeting by the President that Mr. Cald- 

 cleugh, elected that evening, was on the point of leaving England for 

 Mexico, and would consequently have no other opportunity during 

 this session of attending for admission, Mr. Caldcleugh was allowed 

 to sign the Obligation in the Charter Book, and was admitted a Fel- 

 low of the Society, 



