IV. On the variations of the diurnal range of the magnetic declination as recorded at the Prague observatory
1. The Prague magnetic observations began in July, 1839, and have been continued until the present date. The observation hours, 18h., 22h., 2h., 10h., are common to the whole series, except for the year 1853, during which observations were made only at the hours 8h., 2h., 10h. As far, however, as the estimation of the diurnal range of magnetic declination is concerned, these last three hours are practically as good as the former four, inasmuch as the observations at 22h. are hardly ever made use of in determining the diurnal range. In the determinations herein recorded, magnetic disturbances are included, and the range is a mean monthly one, obtained by comparing together the mean values of the magnetic declination, corresponding to the hours 18h., 22h., 2h., 10h., for any given month, and taking the difference between the highest and the lowest of these values as representing the mean range for that month. There is reason to believe that the ranges thus obtained are not greatly different from those which would have been obtained from an hourly series of observations.