On a special form of magnetic disturbances

On days which are magnetically almost quiet, or only slightly disturbed, we sometimes find in the curves small disturbances of very regular course. Their period is of about one and one‐half to two and one‐half hours, and their chief characteristic feature is, that the horizontal intensity, starting from the quiet march of the curve, is in general gradually increasing or decreasing till it has attained a maximum or minimum value, and then gradually or with small oscillations returns to the undisturbed value, and the curve continues its quiet course. The disturbance appears in declination and vertical intensity, too. In the vertical intensity there is a gradual decrease or increase, and after the extreme values being attained there is a gentle return to the undisturbed value.