Mr Galbralth on tie English Arc of the Meridian. 271 



there is a probability of an error of 7" in each, if considered 

 equal. There is at least, certainly, some inconsistencies in 

 these operations, for which it is difficult to account on any 

 other hypothesis. The effect, however, on the length of the 

 arc of the meridian, would be nearly insensible, though it 

 might in some degree slightly vitiate other deductions, such as 

 the latitudes, longitudes, and azimuths, dependent upon it. 

 Indeed it may be remarked, that the peculiar construction of 

 the old theodolite, by Ramsden, is not favourable to the accu- 

 rate determination of azimuths by the pole-star. The altitude 

 and azimuth circle, or transit instrument properly constructed, 

 would, in my opinion, be greatly superior. A good altitude 

 and azimuth circle, I believe to be the best instrument for de- 

 termining the latitude ; and the adoption of a small arc, as in 

 the case of the zenith sectors, hitherto employed in this country, 

 has always to me appeared not a little singular. A new zenith 

 sector has lately been proposed by Mr Airy, with several im- 

 provements over the old instrument, which, if I am rightly 

 informed, was destroyed by the late fire at the Tower. Still, 

 however, though in the new instrument the angle be read on 

 opposite arcs, yet it seems to be doubtful if its results can be 

 considered equal to those from a circle of much smaller radius, 

 read from three or six microscopes, distributed equidistantly 

 round the circumference, when for every pair of observations it 

 is reversed in azimuth, and the repetitions carried to four or six 

 times within proper limits, and nicely reduced to the meridian. 

 Indeed, notwithstanding the general excellence of the mural 

 circle, as now constructed with microscopes attached to the stone 

 pier for the sake of permanence, yet the circle itself being built 

 up of so many different pieces liable to unequal strain, its ex- 

 ecution is not entirely conformable to sound mechanical prin- 

 ciples.* A transit circle, having both ends of its axis sup- 

 ported on stone-piers, must possess much greater stability, 

 especially if made, with the exception of the axis, of cast-iron, 

 with radiating bars, broad at the axis, and tapering towards 

 the circumference on which the divisions are cut. The glasses 

 of the telescope, too, ought to be much more substantially 



* It has little or no stability by braces in the direction of the axis. 



