1. This paper briefly outlines some results of a study of magnetic storms, the object of which was to ascertain their broad general features, leaving aside individual details and cases. The account is limited in several respects, mathematical developments, in particular, being omitted; this is because the paper is intended to be followed by a more complete discussion, when conditions permit. In this future memoir I hope to deal more adequately with many points here lightly passed over, and especially with disturbance phenomena in polar regions, where magnetic conditions are scarcely ever quiet, and where the divergence between the magnetic and geographical axes of the earth introduces most complication. Part I.— The Magnetic Data. 2. Apparently all great world-wide magnetic storms commence simultaneously to within a few seconds, over the whole earth, although small local magnetic fluctuations may sometimes mask the commencement at particular stations. It is therefore possible, where we are not concerned with very small time intervals, to speak of “storm time,” measured from the beginning of a storm, without reference to any individual locality. In contradistinction to storm time, the same for all stations, is local time, which we shall reckon from local midnight.