Page:Memoirs of a Trait in the Character of George III.djvu/238

NO. 1. cannot be depended upon to do; well therefore might Mr. Maskelyne admit that my Invention would become of considerable value, even if taken in aid of the Lunar tables. I leave the reader to judge of the practicability of making these observations from what follows:

To ascertain the Longitude by the Moon and a star, requires a distinct horizon to be seen in the night, which is next to impossible, and if you have not an horizon, the altitude of neither Moon nor star can be taken: it also requires (and this perhaps when a ship is in a high sea) the distance of the Moon and star, in order to come at which, the image of one of them must be reflected through a silvered glass, and the other seen through an unsilvered part of the same glass; and they must be brought into conjunction in the line that connects the silvered and unsilvered parts, and this to an exactness only true in theory, for an error of a minute of a degree committed in this observation, will mislead the mariner half a degree in his Longitude; now I call upon any Astronomers of reputation publicly to declare, that they have, even at