A comparison of values of the magnetic elements, deduced from the British magnetic survey of 1891, with recent observation

In the preparation of maps showing lines of equal magnetic declination for commercial use from the data given in the last great Magnetic Survey of the British Isles it has of late years become apparent, by comparison with the observations made at the several British Magnetic Observatories of Greenwich, Kew, Stonyhurst, Falmouth, and Yalentia, that, owing to decrease in value of the annual variation, the lines, as brought forward from the survey, are now carried too far to the west, that is to say, they give a value of declination that is too small, the discordance being apparently greater towards the west than on the eastern side. I have thus thought that it would be desirable to examine this question further, not only for declination, but also for horizontal force and dip, comparing the survey values as brought forward to the epoch 1906.0 by use of the annual rates of change which obtained when it was made with the results of recent observation at the places mentioned. It will be interesting, in the first instance, to ascertain in what degree the annual variation of the several elements may have changed in recent times. This is shown in Table I, which gives the mean annual variation of each element at Greenwich, Kew, and Stony hurst in three successive periods, the values in the last period being those appearing in Table II, adding thereto corresponding information from Table XIV of the magnetic survey.