Note 63 — Chapter III
Reading edition
This sketch is not pretended to be minutely accurate; it being morally impossible to procure recent observations in every part, from the rapid change that takes place in the quantity of the variation, in one and the same spot. However, the observations that determine the course of the lines in the Atlantic (and which are marked on the sketch) are from observations so late as 1793. The same is to be said of those in the western quarter of the Mediterranean; and those beyond the Cape of Good Hope, to longitude 30° east, are of the year 1789.
It is obvious that a critical knowledge of the quantity of the variation in any particular place, and at a given time, is of less importance to the present question, than that of the bearing of the lines of equal quantities, at any recent period: and this object is, I think, tolerably well obtained, by the materials before me. Any change that may have taken place since 1793, is in favour of a greater quantity of variation, within the limits of Mr. Park’s travels.
In the Atlantic, the increase appears to be about a degree in seven years. In the Indian ocean, less: and in the Red Sea, there seems to have been little alteration between 1762, and 1776.
The lines, as far as they are founded on authority, are continuous; but broken in the parts assumed.