Magnetkal Observatory of Dublin during the Years 1840-43. 93 



rences in the same manner as the errors of observation (to which they are ana- 

 logous) are combined in the calculus 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 we may therefore call the mean disturbance ; and it is 

 evident that its values, at the several hours of the day, and at the several 

 seasons of the year, will serve to measui-e the probable disturbance to be ex- 

 pected at the corresponding times. 



The values of this function have been deduced for the several hours of ob- 

 servation, in each month of the year 1843 ;* and those for the entu-e year are 

 obtained from them by a repetition of the same process. They are given in 

 the following Table : — 



Table XIII. Values of the Mean Disturbance. 



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 ; and it then decreases with the same regularity, 

 and arrives at its constant day- value about 6 a.m. 



* I have chosen this year, because in it the irregular changes were comparatively small ; and, 

 the number which e.xpresses their frequency, in consequence, bearing a larger proportion to that 

 which denotes their magnitude, any regular law to which they are subject will be more readily 

 apparent. 



