Whoever imagines he can find the variation within a degree, will very often see himself much deceived. For, besides the imperfection which may be in the construction of the instrument, or in the power of the needle, it is certain that the motion of the ship, or attraction of the iron-work, or some other cause not yet discovered, will frequently occasion far greater errors than this. That the variation may be found, with a share of accuracy more than sufficient to determine the ship's course, is allowed; but that it can be found so exactly as to fix the longitude within a degree, or sixty miles, I absolutely deny.[93] [Footnote 93: Few readers, it is presumed, require to be informed, that the mode of endeavouring to ascertain the longitude by the variation of the compass is no longer in use. In a work already referred to, Clerke's Prog. of Mar. Disc., a singular enough communication is inserted respecting the effect of tallow on the compass. It is subscribed by Lieutenant Mason of the marines; but whether the experiments it relates have been repeated by others, or if the inference it maintains has been otherwise confirmed, the writer has yet to learn. He thought it right, however, to notice it, as the more extensively hints are spread which concern the advancement of useful discovery, the greater chance we have of correcting errors, and perfecting science, The same reason justifies his remarking, that the most important observations respecting the variation of the compass made of late years, are those of Captain Flinders, as to the effect of the ship's course upon it. The reader will find them in the appendix to the account of his voyage lately published, 2d volume. Similar observations have still more recently been made by an officer on board his majesty's ship Sibyl, while in the North Sea protecting our Greenland fishery. They form an appendix to the Account of a Voyage to Spitzbergen, by Mr John Laing, Surgeon, published at Edinburgh, 1815. Of their importance and accuracy, notwithstanding the small scale on which they were made, and the meagre manner in which they have been communicated, it is impossible for a moment to doubt. The concluding remark is entitled to considerable regard.--"After a more enlarged series of observations shall have been taken, and after the attention of astronomers is directed to this fact, one may confidently expect a most important improvement in the science of navigation." The value of the discovery alluded to, will at once appear from what is said in the way of enquiry as to similar observations to those made in the North Sea applying to ships coming from the Baltic, viz. that if so, "they most effectually account for ships getting down on the coast of Holland, when they suppose themselves well over in Mid-channel; and therefore prove the loss of so many of our brave tars when coming from that sea."--P. 163. As a paper, containing Captain Flinders's