PHILOSOPHICAL TRANSACTIONS. 



I. On the Laws of the Tides on the Coasts of Ireland, as inferred from an extensive 

 series of observations made in connection with the Ordnance Survey of Ireland. 

 By G. B. Airy, Esq., F.R.S., Astronomer Royal. 



Received November 30, 1844, — ^Read December 12, 1844. 



In the spring of 1842 I was informed by Colonel Colby, R.E., Director of the 

 Trigonometrical Survey, that in the operations of the Survey of Ireland it had become 

 necessary to adopt a line of reference for the elevations ascertained in the running 

 of various lines of level through the country ; and that it was his intention to institute 

 a series of observations of the height of the water in different states of the tide, in 

 order to refer the levels to the mean height of the sea, or to its height at some definite 

 phase of the tide. Colonel Colby stated also that he was desirous that the observa- 

 tions should be made subservient to improvements in the theory of the tides, and 

 requested my assistance in sketching a plan of observation which would be most likely 

 to contribute to that end. 



In reply, I made the following suggestions : — That great care should be taken in 

 the accurate determination of time at every station, and that for this purpose the non- 

 commissioned officer of the Royal Sappers and Miners who had the care of the 

 observations at each station, should be entrusted with a pocket chronometer, and that 

 an officer should, at least twice during the series of observations, visit every station, 

 carrying, for comparison, an itinerant chronometer whose error on Greenwich time 

 was accurately known from astronomical observations. That stations should be 

 chosen on the eastern as well as on the western coast, in order to determine the 

 difference of level, if any, between an open sea and a partially inclosed sea. That on 

 the north-eastern coast, stations should be selected at smaller intermediate distances 

 than at other parts of the coast, with the purpose of removing, if possible, the doubt 

 which appears to exist as to the progress of the semidiurnal tide-wave through the 

 North Channel. That, where practicable, several stations should be selected on each 

 of the large rivers or estuaries, in order to ascertain the nature of the modification 



MDCCCXLV. B 



