HISTORY OF PHYSICAL ASTRONOMY.

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degree was now almost exactly reconcilable with the ellipticity derivedfrom a comparison of meridional arcs in the northern hemisphere. Healso completely succeeded in tracing the various circumstances whichconspired to vitiate Lacaille’s arc. "We have already mentioned that the' error was reduced to half its original magnitude by the remeasurement ofi the terrestrial arc. The remaining half was found by Mr. Maclear toarise from errors in the latitudes of the two extreme stations, occasionedby the attraction of the mountains in their neighbourhood. When thelatitudes were determined at stations removed from the influence of localattraction, and then connected trigonometrically with the arc, they werefound to be very nearly reconcilable with the usually received value ofthe ellipticity. A slight deviation of the plumb-line took place at thesouthern station, but a much greater deviation was occasioned by the at-I traction of the mountains at Kleip Fonteyn. The errors at both stations,j however, conspired together in reducing the amplitude of the arc, agree-ably to the remark of Colonel Everest. Mr. Maclear has earned for him-self a high place among living astronomers by the ability with which hehas executed this important geodetic operation. It is gratifying to reflectthat his labours have removed a serious difficulty from the science ofastronomy, since we are now assured that the actual measurement ofmeridional arcs on each side of the equator concurs with the theory ofgravitation, and the experiments with the pendulum, in assigning the sameellipticity to both hemispheres.

The arc of the meridian connected with the trigonometrical survey ofGreat Britain now extends from Dunnose, in the Isle of Wight, to Balta,one of the Shetland Isles , and embraces an amplitude of 10° 7' 55".28.The latitudes were determined with Ramsden’s zenith sector at ten dif-ferent places, including the two extreme stations. When the whole courseof triangulation was executed, the latitudes at the various stations werethen computed by means of an assumed value of the ellipticity, and therelative bearings and distances, setting out from Greenwich, with whichthe operations were trigonometrically connected. The latitudes found inthis manner for the two extreme stations, on being compared with thosedetermined with the zenith sector, presented a discordance which seemedto imply an error either in the observations, or in the assumed value of theellipticity. The latitude of Dunnose was again determined with the newOrdnance zenith sector*, but no error was discovered in the previous mea-surement. Another station was then selected about a mile distant, andthe latitude, on being determined with the same instrument, was found todiffer as much as 3T2 from the result obtained at the original station.This circumstance is the more remarkable, as the surrounding countrydoes not seem to indicate the existence of any disturbing influence. Thelatitude as thus determined at the new station coincided exactly with thatpreviously found by a geodetic process, and thereby afforded an importantconfirmation of the value of the ellipticity upon which the latter resultdepended. The computed and observed latitudes of the northern extre-mity of the arc at Balta presented also a similar discordance, and weresimilarly reconciled together by repeating the observations with the zenithsector at a place in the neighbourhood of the original station. The me-thod of determining the latitude of a place by connecting it trigonome-

* The superb instrument of Ramsden, having been deposited for safety in the ar-moury of the Tower, was unfortunately destroyed by the fire which consumed that partof the building in 1841.