215 



question, in his able discussion of the Toronto observations, 

 and has arrived at conclusions for the most part confirmatory 

 of those obtained by Professor Kreil. 



In these investigations, however, those disturbances only 

 are taken into account which exceed a certain arbitrary limit ; 

 and, even of these, the frequency is considered without any 

 reference to their magnitude. In examining the question of 

 the periodicity of disturbances, Dr. Lloyd has thought it ne- 

 cessary to pursue a different course. His method consists in 

 taking the differences between each individual result, and the 

 monthly mean corresponding to the same hour, and combining 

 these differences in the same manner as the errors of observa- 

 tion (to which they are analogous) are combined in the calcu- 

 lus of probabilities. The square root of the mean of the squares 

 of these differences is, in fact, a quantity analogous to the mean 

 error, and which he therefore proposes to call the mean distur- 

 bance ; and it is evident that its values, at the several hours of 

 the day, and at the several seasons of the year, are measures 

 of the probable disturbance to be expected at the corresponding- 

 times. 



The values of this function have been deduced for the 

 several hours of observation, in each month of the year 

 1843 ; and those for the entire year are obtained from 

 them by a repetition of the same process. These numbers 

 show that the mean disturbance follows a law of remarkable 

 regularity in dependence upon the hour. During the day, 

 i.e. from 6 a.m. to 6 p.m., it is nearly constant ; at 6 p.m. 

 it begins to increase, and arrives at a maximum a little after 

 10 P.M.: it then decreases with the same regularity, and 

 arrives at its constant day-value about G a.m. 



The preceding results are independent of the direction 

 of the disturbance. If, however, we take the sum of the 

 squares of the easterly and westerly deviations separately, we 

 find that the easterly disturbances preponderate during the 

 night, and the ivesterly during the day ; the former being, 



