How great that precision may be I must endeavour to explain. The latitude of every important observatory is obtained from a large number of observations, and it would be unlikely that it was more than one or two-tenth's of a second different from the actual mean value. Now a tenth of a second on the surface of the earth corresponds to a distance of about ten feet, and this means that the latitude of the observatory or, as we must now speak very precisely, the latitude of the centre of the meridian circle in the observatory, is known to a degree of precision represented by a few paces. It will thus be seen that, with the accuracy attainable in our modern observation, it would often be an appreciable blunder to mistake the latitude of one wall of the observatory for that of the opposite wall; in other words, we know accurately to within the tenth of a second, or within not much more than the tenth of a second, the distance from the centre of the transit circle at Greenwich down to the earth's Equator. But, of course, the distance from the Pole to the Equator is 90, and this being so it follows that the distance from the North Pole of the earth to the centre of the transit circle at Greenwich Observatory has been accurately ascertained within one or two tenths of a second. If any change took place in the distance between the Pole and the meridian circle at Greenwich, then it must be manifested by the changes of latitude. We shall now be able to understand how any movement of the Pole, or rather of the position which it occupies in the earth, would be indicated at Greenwich.
Suppose, for instance, that the Pole actually advanced towards Great Britain, and that it moved a distance of, let us say, thirty feet, the effect of this would be to
