The Rev. H. Lloyd on the Meteorology of Ireland. 419 



of the results. It was arranged, accordingly, that the observations should be 

 continued at the coast-guard stations until the end of the year 1851, so as to 

 embrace a period of at least one year reckoned from the time when the obser- 

 vers had acquired tlie power of observing with accuracy. The monthly means 

 for this year may be reduced to their absolute mean values, by the help of the 

 more extended series of observations made in Dublin, by which the deviations 

 of any monthly result from its absolute mean value is sufficiently known. 



The Committee, upon whom the duty of superintending these arrangements 

 devolved, were desirous that the plan of observation should be the least one- 

 rous that could lead satisfactorily to the results aimed at. One of the princi- 

 pal of these — the determination of the movements of masses of air, whether 

 in storms, or in the displacement of atmospheric waves, — demands, as has 

 been said, that the observations should be taken at equal intervals of time ; 

 and the only condition imposed by the other meteorological problems is, that 

 these times should be so chosen as to furnish the daily means of the elements 

 sought. Now any three observations, taken at equal intervals throughout the 

 day, are sufficient to eliminate the diurnal variation, and therefore to give the 

 daily means of all the meteorological elements ; and undoubtedly, where such 

 a system is practicable, the observations should be taken at 6 a. m., 2 p. m., and 

 10 p. M., which has been shown to be preferable to any other eight-hourly group 

 for meteorological purposes.* 



At the coast-guard stations, however, such a plan of observation would have 

 been incompatible with the regular duties of the men ; and it was advisable to 

 adopt a less complete system, which might be followed at all the stations, and 

 in which interruptions were not likely to occur. Fortunately, two observations 

 in the day, taken at equal intervals, are sufficient to give the daily means of all 

 the meteorological elements, excepting the atmospheric pressure ; and, as the 

 diurnal variation of the pressure is very small, — much smaller than its irregular 

 fluctuations in these latitudes, — it may be disregarded, and the objects for which 

 the present system was instituted may be attained by taking two observations 

 in the day, at homonymous hours. 



* Transactions of the Royal Irish Academy, voL x.xii. p. 65. 

 VOL. XXIL 3 I 



