324 ON THE METEOROLOGY OF IRELAND. 



arrangements devolved, were desirous that the plan of observation 

 should be the least onerous that could lead satisfactorily to the 

 results aimed at. One of the principal 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 mete- 

 orological 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. 



The best pair of homonymous hours, for the determination of 

 the mean temperature, and nearly also for that of the mean humidity, 

 are 9 h 46 m A.M., and 9 h 46 m P.M. Limiting themselves to the exact 

 hours, the Committee might accordingly have chosen either 9 A. M. 

 and 9 P.M., or 10 A.M. and 10 P.M. ; the former pair was adopted, 

 its superior convenience seeming to outweigh the advantage of the 

 latter in accuracy. 



For the fuller elucidation of some of the questions proposed, it 

 was further arranged that hourly observations should be taken at all 

 the stations for twenty-four hours, at the equinoxes and solstices, 

 according to the plan laid down by Sir John Herschel. It was 

 likewise provided, that hourly observations should be taken occa- 

 sionally, under special circumstances, such as storms, unusual dis- 

 turbances of barometric equilibrium, &c. 



