18 COSMOS. 



lini, almost entirely under the so-called mean parallel of 45*. 

 The numerous pendulum experiments which have been con- 

 ducted in the neighbourhood of mountain chains, have con- 

 firmed in the most remarkable manner the previously-recog- 

 nised influences of those local attractions which were in- 

 ferred from the comparison of astronomical latitudes with 

 the results of geodetic measurements. 9 



In addition to the two secondary methods for the direct 

 measurement of a degree on meridian and parallel arcs, we 

 have still to refer to a purely astronomical determination of 

 the figure of the earth. This is based upon the action which 

 the earth exerts upon the motion of the moon, or in other 

 words upon the inequalities in lunar longitudes and latitudes. 

 Laplace, who was the first to discover the cause of these 

 inequalities, has also taught us their application by ingeni- 

 ously showing how they afford the great advantage which 

 individual measurements of a degree and pendulum experi- 

 ments are incapable of yielding, namely, that of showing in 

 one single result the mean figure of the earth. 10 We would 



9 Biot, Astr. Physique, t. ii, p. 482, and t. iii, p. 482. A very accu- 

 rate geodetical measurement, which is the more important from its 

 serving as a comparison of the levels of the Mediterranean and Atlantic, 

 has been made on the parallel of the chain of the Pyrenees by Cora- 

 boeuf, Delcros, and Peytier. 



10 Cosmos, vol. i, p. 160. "It is very remarkable that an astronomer 

 without leaving his observatory, may merely by comparing his obser- 

 vations with analytical results, not only be enabled to determine with 

 exactness the size and degree of ellipticity of the earth, but also its 

 distance from the sun and moon results that otherwise could only be 

 arrived at by long and arduous expeditions to the most remote parts 

 of both hemispheres. The moon may, therefore, by the observation of 

 its movements render appreciable to the higher departments of astro- 

 nomy, the ellipticity of the earth, as it taught the early astronomers 

 the rotundity of our earth by means of its eclipses" (Laplace, Expos, du 

 Syst. du M&iide, p. 230). We have already in Cosmos, vol. iv, pp. 481 

 532, made mention of an almost analogous optical method suggested by 

 Arago, and based upon the observation that the intensity of the ash- 

 coloured light, that is to say the terrestrial light in the moon, might 

 afford us some information in reference to the transparency of our 

 entire atmosphere. Compare also Airy in the Encycl. Metrop. pp. 189, 

 236, on the determination of the earth's ellipticity by means of the 

 motions of the moon, as well as at pp. 231 235. on the inferences which 

 he draws regarding the figure of the earth from precession and nutation. 

 According to Biot'a investigations, the latter determination would only 



