Royal Society. 251 



meanings to some of the Professor's expressions than they 

 might be intended to convey in a popular work. If so, I 

 stand open to correction. At the same time, however, it 

 should be observed, that any mechanical theory only profess- 

 ing a popular character, must necessarily be superseded by 

 investigations more demonstrative, and leading to more deter- 

 minate conclusions. 



1 am, Gentlemen, 



Your obedient Servant, 

 Cambridge, February 17, 1845. W. HoPKINS. 



XXXVI. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



Dec. 12, " f\^ the Laws of the Tides on the Coast of Ireland, as 

 1844. ^"^ inferred from an extensive series of observations 

 made in connexion with the Ordnance Survey of Ireland." By George 

 Biddell Airy, Esq., F.R.S., Astronomer Royal. 



The elaborate investigations of which the results are communi- 

 cated in the present paper, were suggested by the necessity of 

 adopting some standard mean height of the sea, as a line of refer- 

 ence for the elevations ascertained in the operations of the Ordnance 

 Survey of Ireland. Colonel Colby, R.E, who conducted that survey, 

 had with this view determined to institute a series of observations 

 on the height of the water in different states of the tide ; and con- 

 ceiving that these observations might be made subservient to im- 

 provement in the theory of the tides, requested the assistance of the 

 author in laying down the plan of observation best calculated to 

 effect that object. The suggestions which were, in consequence, 

 made by the author were adopted in their utmost extent by Colonel 

 Colby ; and the collection of observations was placed in the author's 

 hands in the winter of 1842. The whole number of observations 

 exceeds two hundred thousand ; and they derive extraordinary value 

 from the circumstance of the localities, of their simultaneity, their 

 extensive range, and the uniformity of plan on which they were con- 

 ducted. Their reduction was made by the computers at the Royal 

 Observatory, Greenwich, under the superintendence of the author, 

 and with the authority of the Lords Commissioners of the Treasury. 

 The nature of the different branches of the inquiry may be gathered 

 from the titles of the several sections into which the paper is divided, 

 and which are as follows : — 



Section I. — Account of the stations, levellings, times and methods 

 of observation. 



Section II. — Methods of extracting from the observations the 

 times of higli and low water; of supplying deficient times and 

 heights ; and of correcting the times first determined. 



Section III — Theory of diurnal tide as related to observations 

 only; and deduction of the principal results for diurnal tide given 

 immediately by these observations. 



S2 



