The Rev. 11. Lloyd on the Mean Results of Observations. 67 



which coincide, abnost exactly, with the best hours for the determination of the 

 mean temperature. 



By taking the mean of the declinations observed at any four equidistant 

 hours, as the mean of the day, the limit of error is reduced to 0'-30. 



9. It appears from the preceding, that any three equidistant observations 

 are sufficient to give the daily mean values (and, therefore, also the monthly 

 and yearly mean values) for each of these elements, with nearly the requisite 

 precision ; and that, by a suitable choice of the hours, the degree of accuracy 

 may be augmented as much as we please. But, in determining the parti- 

 cular hours for a continuous system of observations, this should not be made 

 the primary ground of selection. The error of the daily means being in all 

 cases reduced within narrow limits by the method already explained, we should 

 choose the particular hours which correspond nearly to the maxima and minima 

 of the observed elements, so as to obtain also the daily ranges. This condition 

 will be fulfilled in the case of the magnetic declination, very nearly, by the hours 



6 A. M., 2 p. M., 10 p. M. ; 



which will, moreover, give nearly the maximimi and minimum of temperature, 

 and of the tension of vapour, together with the maximum pressure of the gaseous 

 atmosphere.* And, if we add the intermediate hours, 10 a. m. and 6 p. m., we 

 shall have, nearly, the principal maxima and minima of the two other magnetic 

 elements. Accordingly, for a limited system of magnetical and meteorological 

 observations, at places for which the epochs of maxima and minima do not 

 differ much from those at Dublin, the best hours of observation appear to be 



6 A. M., 10, 2 p. M., 6, 10. 



The conditions of the problem are altered, if at any place the laws of the 

 diurnal variation have been already obtained from a more extended system of 



* The ternary combination above proposed possesses the further advantage of coinciding, 

 nearly, with one of those deduced above, as the most favourable for the determination of the mean 

 temperature and mean declination. The errors of the resulting means are found by making 

 X = 90° in the third terms of the general formula ; and we thus find the error of temperature 

 = - <f-Ql, while that of the declination = - 0'-20. 



k2 



