.noJ^a-inaioH. V a«a\*i v ^Jo^a/ Society. 129 



which he instituted with glass gratings. I experimented also at 

 first with these, and at times I thought I had obtained the direc- 

 tion of vibration as Stokes had found it ; but I could never find 

 any regularity. The glass gratings, that is, such as are produced 

 by incisions with the diamond in glass, are not suitable for 

 this purpose, because the irregular surface of the section com- 

 plicates the phsenomenon. To this circumstance I ascribe the 

 error of Stokes. 



XXI. Proceedings of Learned Societies. 



ROYAL SOCIETY. 

 [Continued from p. 76.] 

 May 8, 1856. — The Lord Wrottesley, President, in the Chair. 



np'HE following communicatiou was read : — 

 -*- " On the Figure, Dimensions, and Mean Specific Gravity of the 

 Earth, as derived from the Ordnance Trigonometrical Survey of Great 

 Britain and Ireland." Communicated by Lieut. -Colonel James, R.E., 

 F.R.S. &c., Superintendent of the Ordnance Survey. 



The Trigonometrical Survey of the United Kingdom commenced 

 in the year 1 784, under the immediate auspices of the Royal Society ; 

 the first base was traced by General Roy on the 16th of April of 

 that year, on Houuslow Heath, in presence of Sir Joseph Banks, 

 then President of the Society, and some of its most distinguished 

 Fellows. 



The principal object which the Government had then In view, was 

 the connexion of the Observatories of Paris and Greenwich by means 

 of a triangulation, for the purpose of determining the diff"erence of 

 longitude between the two observatories. 



A detailed account of the operations then carried on is given in 

 the first volume of the ' Trigonometrical Survey,' which is a revised 

 account of that which was first published in the ' Philosophical 

 Transactions' for 1785 and three following years. 



At the time when these operations were in progress, the Survey of 

 several counties in the south-east of England, including Kent, Sussex, 

 Surrey, and Hampshire, was also in progress, under the direction of 

 the Master-General of the Ordnance, for the purpose of making 

 military maps of the most important parts of the kingdom in a mili- 

 tary point of view ; and it was then decided to make the triangula- 

 tion which extended from Hounslow to Dover the basis of a trian- 

 gulation for these surveys. 



It is extremely to be regretted that a more enlarged view of the 

 subject had not then been taken, and a proper geometrical projection 

 made for the map of the whole kingdom. As it is, the south-eastern 

 counties were first drawn and published in reference to the meridian 

 of Greenwidh, then Devonshire in reference to the meridian of I'ut- 

 terton in that county, and thirdly the northern counties, in reference 

 to the meridian of Delamere in Cheshire ; but there is a large inter- 



Phil. May. S. 4. Vol. 13. No. 84. Feb. 1857. K 



