Numerical character‐figures of magnetic disturbance in relation to geomnagetic latitude

§1. In recommending the publication of daily values of the quantity Cn= (HRH+ZRZ)×10−4 at a number of selected observatories the primary intention of the responsible sub‐committee of the International Union of Geodesy and Geophysics was to provide a numerical criterion for differentiating between days of greater or less magnetic disturbance at any one station. At best Cn has only a facial resemblance to one of the constituents in the expression for the total energy of the Earth's magnetic field. Without complex modifications to take account of the contributions to the quantity made by the regular quiet‐ and disturbed‐day variations Cn can hardly be regarded even as a rigorous comparative measure of the major features of disturbance at various observatories. To serve this purpose it would require to be shown that not only the same type of perturbation dominated disturbance at all observatories, so that the extreme daily range R and not, say, the mean hourly range, could be regarded as a universal index of disturbance, but that the dominant perturbations occurred at least within the same Greenwich day at all observatories.