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 publickly to declare, that they have, even at Land, and with the best Instruments _Europe_ affords, been able to make this Observation of the Moon and a Star with _any thing like_ the Precision required to determine the Longitude within the Limits required by the Act of the 12th of Queen _Anne_; I know it cannot be done. Nay I further call upon any such Astronomers to declare, whether even in Observations of the Distance between the Sun and Moon, two of them observing together have _generally speaking_ agreed in this Observation within a Minute of a Degree: I know that in general the Difference between the best Observers even at Land will be more, and as a farther Proof of this Assertion, I refer the Reader to the Note below:[6] And if these Matters of Fact are still doubted, I shall beg Leave to call upon Mr. _Maskelyne_ and Mr. _Green_ to declare how near they, with Admiral _Tyrrel_ agreed in determining the Longitude by the Sun and Moon in their Voyage to _Barbadoes_; and also whether during that Voyage they ever did determine their Longitude by the Moon and Stars.—I know they did not, for they found the Observation too difficult, and indeed _it is only true in Theory_.
1st. That during six Days in every Month, no Observations can be made by this Method to ascertain the Longitude at Sea.
2dly, That during 13 other Days in each Month, it is impracticable to ascertain it by this Method with any Instruments hitherto contrived, or which the Nature of the Service to be performed seems to admit of