at the Magnetic Observatory at Prague. 



427 



during the remaining hours of the day, the perturbations may 

 so little exceed the average, that on the whole it may not be 

 a day of disturbance according to the above definition. 



This phenomenon, which may be called a magnetic shock, 

 ought to be brought into notice, being, in fact, a perturbation 

 of short duration, greater perturbations consisting only of 

 several such shocks. As a definition of a magnetic shock, 

 let be the change in either of the magnetic elements be- 

 tween two successive observations, and <r the average change 

 in the same month between every successive pair of observa- 

 tions ; then every change for which -- > 2, is to be regarded 



as a magnetic shock. 



The following table shows. for each month the number of 

 days of disturbance according to the above definition ; and 

 also the number of separate shocks which took place in addi- 

 tion, namely, on days which were not those of disturbance. 



Table of Disturbances. 



From this table we may infer: 



I. That the perturbations are much more frequent in the 

 winter than in the summer months ; which may be caused, in 

 part, by the force which produces the regular diurnal changes 

 being much weaker in winter : but the very great perturba- 

 tions which take place chiefly in the winter months, indicate 

 that the disturbing forces have actually more intensity at that 

 season of the year. The greatest disturbances observed in 

 the twelvemonth occurred on the following days: 



