horizon H—O. When any fixed star, X, has not exactly the same right ascension as any other fixed star, Y, the culmination of the fixed star X can be found just as if it had the same right ascension as the fixed star Y: for, when the fixed star X is on the meridian, the deflection of the fixed star Y, from the perpendicular at the fixed star X, will be accurately the difference in right ascension between the fixed stars X and Y, and vice versa.[1]
- ↑ The method here presented, for ascertaining the meridian of any observer, will be found accurate, within limits so very narrow, as at first view to appear incredible: but when it is considered, that this method is no way affected by refraction or by parallax, the former of which constitutes the grand natural obstacle to accuracy in all astronomical observations which are made according to the methods heretofore practised; and the latter, on account of its intimate connection with the former, presents serious difficulties; I say, when it is considered, that the method here presented is not clogged by those insuperable impediments, doubt will subside; and according as observations are made pursuant to this method, which substitutes the simplicity of nature for the complexity of art, doubt will give place to confidence. That this method is no way affected by refraction or by parallax is clear; because it does not require any recurrence to the elevation or to the depression of the celestial bodies; and as light moves only in right lines, the position of those bodies can be affected only with respect to their elevation and to their depression by refraction: and the fixed stars, which are the only celestial bodies to which this method has a primary regard, have no sensible parallax.
The method now in use for ascertaining the meridian of an observer, by the altitudes of any of the celestial bodies above the horizon of that observer, is attended by several impediments, which constitute an insurmountable bar to the accuracy which most observations demand. Every person, however slightly conversant in those subjects, knows, that the refractive powers of any medium is proportionate to its density; and that, of all media, the atmosphere is subject to the greatest fluctuations: those fluctuations are very numerous, and many of them have hitherto eluded the utmost effort of investigation, though aided by the nicest experimental tests. And, besides, the causes of many of them are wholly unknown: and, again, the causes of some of them, though known as to their general operation, cannot be traced in particular instances, in which it often happens, that the knowledge of them is most wanting: so that to say when the refractive power of the atmosphere is a maximum (or at its greatest power), or when it is a minimum (or at its least power), has been hitherto found absolutely impossible: and the knowledge of its intermediate powers, for the same reasons, is so obscure as to
