of the year, but with that of the lunar month. The daily oscillation also varies at times in a sudden and irregular manner. The needle has been exhibiting, for several weeks, the most perfect uniformity of oscillation. Day after day the careful observation of the needle's progress has revealed a steady swaying to and fro, such as may be seen in the masts of a stately ship at anchor on the scarce-heaving breast of the ocean. Suddenly a change is noted; irregular movements
Fig. 7.
Changes of Appearance of the Great Sun-Spot of 1865.
become perceptible which are totally distinct from the regular periodic oscillations. A magnetic storm is in progress, and its progress does not affect only the place of observation, but widely-extended regions of the earth; and in some well-authenticated instances these magnetic vibrations thrill in one moment the whole frame of the earth."
