183 



of pressure. The author accordingly proposes, as the best 

 hours of observation in a limited system, 



6 a.m., 10, 2 P.M., 6, 10. 

 The case is different where the course of the diurnal curve 

 has been already obtained from a more extended system 

 of observations. In this case the mean of the day may be 

 inferred from observations taken at any hours whatever ; and 

 the hours of observation should therefore be chosen, chiefly, 

 if not exclusively, with reference to the diurnal range of the 

 observed elements. 



The author proceeds, in the next place, to consider the 

 course to be pursued in the reduction of a more extended sys- 

 tem of observations (such as that prescribed by the Royal So- 

 ciety in 1839, and adopted by all the Magnetical Observatories), 

 when some of the observations are deficient. He shows that, 

 in this case, in deducing the daily means from the remaining 

 observations, we must attend, not only to the elimination of 

 the regular diurnal variation, but also to that of the irregular 

 changes of longer periods, which are sometimes (as in the case 

 of the atmospheric pressure) more influential in the result. With 

 this view he determines the values of the mean daily fluctua- 

 tion for each of the elements already referred to ; and com- 

 pares the mean values of the horary changes thence arising 

 with that resulting from the regular diurnal variation. 



The author shows, finally, in what manner the monthly 

 means of the results obtained at any hour are to be corrected 

 in the case of deficient observations, so as to render them com- 

 parable with those in which none are wanting ; and he deduces 

 the probable values of these corrections for each element, 

 with the view of ascertaining in what cases the correction may 

 be disregarded, and in what it is indispensable. 



Professor Graves exhibited and described a silver brooch, 

 belonging to the Royal Dublin Society, and bearing on it an 

 inscription in the Ogham character. 



