Periodicity as to days seems to have no certain law; and though certain days in February and March are marked as those of fine returning displays, they must be looked on as accidental. [Sidenote: Maxima and minima.] Two well-marked annual maxima seem to occur in March and October (the latter the greater), and two minima in June and January, the greater in June (Encyc. Brit.). The 4th of February, 1872, and same day 1874, are, however, curious instances of a recurring remarkable display. [Sidenote: Kæmtz’s table.] A table by Kæmtz, showing the number of Auroras in each month of the year, with the maxima and minima as above stated, will be found on Plate V. fig. 5. [Sidenote: Dr. Hayes’s observations in winter of 1860-61.] Dr. Hayes has observed that in the winter of 1860-61 (when the ten or eleven years’ inequality was at its maximum) only three Auroræ were seen and recorded, and they were feeble and short in duration. [Sidenote: Captain Maguire’s observations at Point Barrow as to number and time of appearances.] Captain Maguire, at Point Barrow (1852-54), reports that the Aurora was seen six days out of seven, and on 1079 occasions, being nearly one third of the hourly observations. It was seldom seen between 9 A.M. and 5 P.M., not at all between 10 A.M. and 4 P.M. It increased regularly and rapidly from 5 P.M. until 1 A.M., and then diminished in the same way until 9 A.M. The winters of 1877 and 1878 and the springs of 1878 and 1879 have been singularly deficient in Auroræ. I have seen none at Guildown. _Duration of Aurora._ [Sidenote: Duration of Aurora. Sometimes a few minutes; at other times the whole night or even days.] In the article in the ‘Edinb. Encyc.’ before referred to some remarks are made on the duration of the Aurora. Sometimes it is formed and disappears in the course of a few minutes. At other times it lasts for hours or during the whole night, or even for two or three days together. Musschenbroek observed one in 1734 which he considered to have lasted ten days and nights successively, and another in 1735 which lasted from the 22nd to the 31st March. [Sidenote: Auroræ may run on into the day without being noticed.] With respect to Captain Maguire’s observations (_antè_) it may be remarked that Auroræ may doubtless frequently run on into and through the day without their being noticed (instances, however, are known of Auroræ seen in daylight); and hence it is difficult to judge of the limit of duration of a particular Aurora unless indications are sought for during the day (by the shapes of clouds, action of the magnet, &c.) as well as during the night. Probably Auroræ seen during successive nights may be parts of a continuous discharge. _The Travelling of Auroræ._ [Sidenote: Travelling of Auroræ. Donati’s investigations.]