To find the Time:—When one line, or any number of lines, of fixed stars culminate, that is, come to the meridian, let a good common watch, or watches, be then set as to noon, or twelve o'clock, for each line of stars respectively; and the time indicated by that watch, or those watches, when the same line, or lines, of fixed stars is, or are, on the same meridian the following evening, plus the sun's (or more properly speaking the earth's) motion during twenty-four hours, converted into time, will give a result so very near the true time, as to leave scarcely any error capable of affecting the observation for the longitude[1].
- ↑ Let the fixed stars, Alpharetz, α, in the head of Andromeda, and
render results which are affected by atmospherical refraction extremely doubtful.
If this were the proper place, it would be quite easy to show, that the labours of the closet on the subject of atmospherical refraction, however aided by experiment and adorned with the specious intricacy of theoretical calculus, have always some latent assumption mixed up with them; and not only so, but that experiments made with the greatest care, at the same time of the same day, in the same latitude and temperature, &c., but not in the same place and with the same identical instrument, never indicate the same results: and further, that even under all the same circumstances, as nearly similar as possible, amongst several observations scarcely any two of them will indicate the same results; so that after all the labour which has been expended, atmospherical refraction remains still—a fugitive principle, and the observations affected by it remain shaded by obscurity and by doubt.
The difficulty of ascertaining the horizon with tolerable accuracy is another insurmountable impediment to finding a meridian within the necessary limits. Several days pass, during which the horizon remains undefined, in consequence of clouds, mists, &c.; and even when the firmament is quite clear and the weather calm, the undulations of the sea, the motion of the vessel, the dip of the horizon, &c. are adverse to accuracy of observation; insomuch, that seamen are known frequently to take an average for the dip, and even to lump, as they term it, the dip and the refraction! On land, not bordering on the sea, any mode of attempting to ascertain the horizon amounts almost to absurdity. Another obstacle to ascertaining the horizon is parallax, which, though in itself it is ascertainable, admitting the earth to be a perfect sphere, yet as the observation for parallax is necessarily affected by refraction, it must, inevitably, partake of the uncertainty which is always attendant on refraction: other considerations might easily be adduced, but more would appear unnecessary.
