70 The Rev. H. Lloyd on the Mean Results of Observations. 



cannot be eliminated, we must attend chiefly to that which is greater in amount. 

 For the purpose of comparing their magnitude, I have taken the differences of 

 the successive daily means, for the declination, the atmospheric pressure, and 

 the temperature, as deduced from the observations of the year 1843 ; and have 

 calculated the square root of the mean of the squares of these differences. The 

 results, which may be taken as the measures of the irregular changes from day 

 to day, are the following : 



3Iea7i Fluctuation from Day to Day. 



Magnetic declination, . . . Fluctuation = l'-04. 



Atmospheric pressure, -214. 



Atmospheric temperature, 3"-07. 



Similarly, if we take the differences of the yearly means corresponding to the 

 successive hours of observation, and combine them in the same way, we obtain 

 the mean two-hourly fluctuations, arising from the regular diurnal change. 

 These numbers are the following : 



Mean Fluctuation in two Hours. 



Magnetic declination, . . . Fluctuation — 2'-04. 



Atmospheric pressure, -0065. 



Atmospheric temperature, l°-46. 



These numbers, compared with the twelfth part of the former, serve to 

 measure the relative magnitude of the regular and irregular changes to which 

 the elements are subject in the same time. We thus find that, in the case of 

 the magnetic dechnation, the irregular change (which is less than ^Lth part 

 of the regular) may be safely neglected ; and we have only to attend to the 

 diurnal changes, and to the irregular changes of short period. The daily 

 means are, therefore, to be deduced from one of the combinations of Art. 10, 

 giving the preference to that which contains the greatest number of indivi- 

 dual results. 



In the case of the atmospheric temperature, the irregular change (which is 

 less than one-fifth part of the regular) is small ; aud we must attend chiefly 

 to the latter. The mean of the day is, therefore, to be inferred from one of 



