When the portable quadrant was wanted for astronomical purposes, the plane was feed vertically, and it is then usually called an astro nomical quadrant. A great many instruments of this construction were made by Bird, Ramaden, and the Troughtons, in the latter half of the last century, and in careful hands a great des' of work may be done with such a quadrant.* Thus observations of the sun or stars at the same altitude on each side of the meridian will furnish an excellent determination of the time, and zenith distances of stars near the zenith in reversed positions of the instrument (the excess arc, as it is called, affords the means) will yield a good latitude. Observations of northern stars combined with southern stars at similar altitudes will give a very close approximation to the latitude when the trno places of the stars arc taken from a good catalogue. For the mode of adjusting and using the quadrant we must refer to the older books or encyclopeedia,s which treat of astronomical instruments. The infe riority of the quadrant to the entire circle is such that there is no probability of its ever returning into fashion, and we believe there is not a single public observatory in which it is now in use. The single advantage of the quadrant is that the divisions are larger and conse quently more easily read and subdivided than in a circle with the same telescope. But this trifling superiority is much more than com
pensated by the power of reading off the circle at several points and taking the mean. On the other hand it is impossible in the qusdrant to secure the exactness of the total arc, or the concentricity of the centre of motion snd the centre of the divisions, while the necessity of leaving some liberty of motiun to the axis carrying the telescope allows of a little wandering of the centre-work, which is perpetually shifting its place. Thus it was found that in the celebrated Greenwich quad rant, though the error of division was probably not more than I , the uncertainty arising from other causes might easily be 3" or even mom. Again, in the mural quadrant it seems difficult so to support it as to resistthe long continued effort of gravity in altering its form, without at the same time rendering it unstable. The Greenwich quadrant was found to have sensibly altered its shape, that is, it had become flattened about 45*, and pulled out at the two extreme radii, which was shown by the errors in the places of stars observed by it when compared with their places by circular instruments, and also by an actual measurement of the several radii and chords. For more minute information the reader is referred to Lalande's Astronomic, § 2311, &c., 3me. edition ; l'inee's Practical Astronomy, chapter v.