X . NOTES. 



( 24 ) p. 30. Bessel, in a letter to myself, Dec. 1828, says : " It often appears 

 to me as if the Earth's ellipticity was sometimes spoken of as uncertain, because 

 too great exactness is demanded. If we take gfo, 3^, ^ and 3^, the corre- 

 sponding differences between the greater and the lesser radius are 10554, 

 10905, 11281, and 11684 toises. The change of 30 units in the denominator 

 only alters 1130 toises in the polar radius; an amount which, viewed in 

 relation to the visible inequalities of the Earth's surface, appears so immaterial, 

 that I am really often astonished that the experiments should agree within such 

 near limits. Detached observations, isolated in wide spaces, would indeed teach 

 us little more than we know at present ; but it would be important to unite all 

 the measurements over the entire surface of Europe, bringing all the astro- 

 nomically determined points into the operation." Thjs, however, would still only 

 show rtie form of the Earth in what may be termed the western peninsula, or 

 projecting part of the great Asiatic continent, about 66 of longitude. The 

 steppes of Northern Asia, even the central Kirghis Steppe, of which I have seen 

 a considerable portion, are often hilly, and, on the whole, not to be compared in 

 point of horizontality with the Pampas of Buenos Ayres and the Llanos of Vene- 

 zuela. These South American plains being distant from the mountains, and 

 covered with secondary and tertiary strata of very uniform and moderate density, 

 the anomalies which might be presented by pendulum experiments at different 

 parts of their surface would afford very pure, and, therefore, decisive results, as 

 to the local constitution of the deep interior terrestrial strata. Compare my 

 Ansichten der Natur, Bd. i. S. 4, 12, and 4750. 



( 25 ) p. 31. Bouguer, who invited La Condamine to undertake experiments 

 on the deflection of the plumb line by the Chimborazo, does not indeed mention 

 Newton's proposal in his " Figure de la Terre," p. 364 394. Unfortunately, 

 the best informed of the two travellers did not observe at opposite sides (east and 

 west) of the great mountain, but (Dec. 1738) at two stations on the same side, 

 one being south 6l west (distant 4572 toises from the centre of the mountain- 

 mass), and the other south 16 west (distant 1753 toises). The first was in 

 a part well known to me, probably below the height where the small mountain- 

 lake of Yana-Cocha is situated, and the other in the pumice -covered plain of the 

 Arenal. (La Condamine, Voyage & 1'Equateur, p. 68 70.) Contrary to all 

 expectation, the deflection indicated by the star altitudes was only 7" - 5; which 

 was attributed by the observers themselves to the difficulties attendant on ob- 

 servations made so near the limits of perpetual snow, the inexactness of the in- 

 struments, and, above all, to supposed great cavities in the colossal trachytic 

 mountain. I have expressed, on geological grounds, considerable doubts as to the 

 justness of the assumption of these great cavities and consequent small mass of 



