The Rev. H. Lloyd on the Mean Results of Observations. 71 



the combinations of Art. 10, giving the preference to those of Art. 11, whose 

 epoch is noon. 



In the case of the atmospheric pressure, on the contrary, the irregular 

 change (which is triple the regular) is the more important. The mean of the 

 day IS, therefore, to be deduced from any combination whose epoch is noon 

 gmng, however, the preference to one of those of Art. 11, in which the diurnal 

 change is also eliminated. 



13. I now proceed to consider the reduction of the montUy means, in the 

 case of deficient observations. 



For the purpose of determining the regular diurnal variation of any magnetic 

 or meteorological element, it is necessary to take the mean of an adequate 

 number of separate results corresponding to each hour of observation, so as 

 to eliminate the irregular and accidental changes. The results usually so com- 

 bined are those of each month. Their number is, in general, sufficient for 

 the purpose above-mentioned; while, on the other hand, the course of the di- 

 urnal change is sufficiently different from one month to the next, to demand a 

 separate determination. 



But in the case of deficient observations, the monthly means of the results 

 corresponding to each hour will not exhibit, in general, the true course of the 

 diurnal change without a correction. If a result be wanting at one hour of 

 a day, m which all the results are much above the mean, it is obvious that 

 the monthly mean corresponding to that hour will be too small, as compared 

 with the means of the other hours ; while, on the other hand, it will be too 

 great, when all the results of the day in question are below the mean. The error 

 will be greater, the greater the variation of the element observed from day 

 to day. In the case of the atmospheric pressure it is so considerable, that the 

 uncorrected monthly means afford no approximation to the law of the diurnal 

 change, in the case of deficient observations. 



The remedy which first suggests itself, in such a case, is to omit all the results 

 of a day m which one or more are wanting. This process is inartificial and un- 

 satisfactory. The xoeight of the mean is diminished in the proportion of the 

 number of observations combined ; and it is therefore important to employ all 

 the observed results in its deduction, provided we can obtain a correction 

 buch a correction is easily found. 



