It was not till the day before yesterday that, upon coming to the city, I received your favour of the 16th. ulto; and this must be my apology for not replying sooner. The usual course with the Phil. Socy. is to refer every communication to a committee, who report their opinion to the socy, merely on the propriety of publishing the communication in their Transactions: but neither the Socy, nor any committee thereof, ever give their opinion on the merits of any paper laid before them, either to the author, or the public—In compliance, therefore, with your desire, I shall, as an individual, give my opinion of the communication from Capt. Groves.

A telescope proper, for observing the eclipses of Jupiter’s satellites must have a considerable magnifying power; consequently its field of view can be but small; probably not exceeding 20 or 30 minutes of a degree: Any angular motion, therefore, of the instrument, amounting only to this small quantity, must throw the object out of the field of the telescope. Nor will the contrivance, proposed by the Captain, of bringing the image of the object down to the horizon, by two previous reflections, in the least degree, obviate this difficulty: for as every vertical motion of the instrument must vary the angle of incidence, and consequently that of reflection, both at the first and second speculum, the last reflected ray will thus be thrown above or below the axis, or intirely out of the field of the telescope. Besides the loss of light from these two additional reflections would be so great as perhaps to render any telescope whatever inadequate to the purpose of making such observations. But of what advantage, it may be asked, would such an improvement be to navigation? supposing it possible to be effected, which I am sure it is not—Certainly of none at all—for it has been clearly proved, by Mr. Ellicott, and other able astronomers, that the mean of a set of good observations of the lunar distance, will give the longitude with as much accuracy as a single observation of the immersion or emersion of one of Jupiter’s satellites; and as the former observations may be repeated at pleasure as well on sea as on land, the method of finding the longitude by lunar observation has, of consequence, a decided advantage over that by the eclipses of Jupiter’s satellites.

I am, Sir, with the greatest respect & esteem Your most obedient servant