150

HISTORY OF PHYSICAL ASTRONOMY.

trically with a distant station, is probably destined to throw much valuablelight on the subject of local disturbance, especially since the true figureof the earth is now pretty well ascertained.

It will readily appear on the inspection of a map that the position ofthe British Isles is well adapted for the measurement of an extensive arcof parallel. An important illustration of the truth of this remark hasbeen recently afforded by Mr. Airy’s determination of the arc of longitudecomprised between Greenwich and Valentia, a small island on the south-west coast of Ireland *. This island is situated about 11° west ofGreenwich, and lies nearly in the same latitude with it. Mr. Airyproposed in the first instance to determine the exact difference of thelongitudes of the two places by the transportation of chronometers, andthen to effect the same object by means of the hearings and distancesassigned by the Ordnance Survey , combined with certain assumed elementsof the earth’s figure. If the two results were found to agree withinsufficiently narrow limits, it might then he fairly presumed that theelements of the earth’s figure were well determined. On the other hand,if they differed to a sensible extent, the amount of the difference wouldserve to indicate the correction which it would be necessary to apply tothe assumed elements. Feagh Main, in the island of Valentia, wasselected as the extreme western point of operations, while Kingston, nearDublin, and Liverpool, were used as intermediate stations. The elementsof the earth’s figure, assumed as the basis of the geodetic calculations,were those which Mr. Airy had arrived at, by a discussion of arcs of themeridian, and have been already mentioned in this chapter. The com-parison of the chronometrical and geodetic arcs exhibited a most gratifyingaccordance, and thereby afforded a valuable confirmation of the assumedelements of the earth’s figure j. This is assuredly not the least valuableof the many results for which astronomy is indebted to Mr. Airy.

We now proceed to give some account of the researches that have beenprosecuted for the purpose of deducing the ellipticity of the earth from thevariation of gravity at its surface. We have already mentioned that

* Mem. Ast. Soc., vol. xvi.

f The following are the results obtained by a comparison of the various geodetic andchronometrical arcs.

Geodetic arc of longitude from Greenwich to Liverpool . . ]2 m 0 s .35

Chronometrical arc.12 0 .05

Chronometrical arc smaller. 0 .30

Geodetic arc from Greenwich to Kingston.24 m 31 s .48

Chronometrical arc .. 24 31 .20

Chronometrical arc smaller. 0 .28

Geodetic arc from Greenwich to Feagh Main.41 m . 23 s .07

Chronometrical arc.41. 23 .23

Chronometrical arc larger. 0 .16

Geodetic from Liverpool to Kingston 12 m . 31 s . 13

Chronometrical arc.12 31 .15

Chronometrical arc larger. 0 .02

Geodetic arc from Kingston to Feagh Main.16 m 51 s .59

Chronometrical arc.’.. 16 62 .03

Chronometrical arc larger. 0 ,44