File:The Bakerian Lecture- A Magnetic Survey of the British Isles for the Epoch January 1, 1886 (1890) (14780182384).jpg

Identifier: philtrans08603087 (find matches)
Title: The Bakerian Lecture: A Magnetic Survey of the British Isles for the Epoch January 1, 1886
Year: 1890 (1890s)
Authors: Rucker, A. Thorpe, T.
Subjects: Proceedings of the Royal Society of London Philosophical Transactions of the Royal Society
Publisher: Royal Society of London

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Isogonals in South-Eastern England. The point at which the terrestrial and true isogonals would intersect if the districtwere not otherwise disturbed is marked by a dot, which will hereafter be called thefocus of the Reading disturbance, SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY 1, 1886. 283 Let us now turn to the calculated disturbances of the elements. In the accom-panying map (fig. 16) the figures represent the differences between the obseryed andcalculated values of the Declination expressed in minutes of arc, and taken as positivewhen the needle is turned to the west. There is a sharply marked boundary betweenthe regions of positive and negative disturbance which passes through the focus. Tothe west of it the needle is deflected to the east and vice versa. Fig. 16.
Text Appearing After Image:
Declination disturbances in minutes of arc.-f- Indicates that the observed westerly Declination is greater than the calculated value. Curve of no disturbance. To the south of the focus the needle is oppositely deflected on each side of a linewhich runs nearly due north and south. To the north the effect on the Declinationneedle dies out. If the cause of the disturbance were a mass of magnetic matter below thesurface of the Earth symmetrical with respect to the isogonals, the line which dividedeasterly from westerly disturbances of the Declination would intersect that whichdivided positive from negative disturbances of the Horizontal Force over the focus orcentre. The next map (fig. 17) shows that this condition is very nearly fulfilled, thetwo points being only 7 or 8 miles apart. Lastly, the maximum disturbance of the 2 o 2 284 MR. A. W. RITCKER AND DR. T. E. THORPE ON A MAGNETIC Vertical Force should occur at the same point, and fig. 18 shows that we find it to beat Reading, which i

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