NOTES. XI 



the trachytic dome of Chimborazo. To the south-south-east of Chimborazo, 

 near the Indian village Calpi, is situated the eruption- cone of Yana-Urcu, which 

 I examined carefully with Bonpland, and which is certainly of more recent 

 origin than the elevation of its giant dome-shaped neighbour. We found nothing 

 resembling a crater in the latter. See the ascent of Chimborazo in my " Kleinere 

 Schriften," Bd. i. S. 138. 



( M ) p. 31. Baily, Esper. with the Torsion Rod, for determining the mean 

 Density of the Earth, 1843, p. 6; John Herschel, Memoir of Francis Baily, 1845, 

 p. 24. 



(") p. 32. Reich, neue Versuche mit der Drehwage (New Experiments with 

 the Torsion Balance), in the Abhandl. der mathem. physischen Classe der Kb'n.- 

 Sachsischen Gesellschaft der Wissenschaften zu Leipzig, 1852, Bd. i. S. 405 

 and 418. The latest experiments of my excellent friend Reich are rather nearer 

 the fine investigation of Baily. I have derived the mean (5*5772) from the 

 series of experiments a and b : a. with the tin sphere and the longer and thicker 

 copper wire, 5*5712, with a probable error of 0'0113; b. with the tin sphere, and 

 the shorter and thinner copper wire, as well as with the tin sphere and the bifilar 

 iron wire, 5'5832, with a probable error of 0'0149. Taking into account these 

 probable errors of a and 6, the mean is 5'5756. Baily's result (5'660), obtained 

 indeed by more numerous experiments, might yet, however, give a rather too 

 high density, as its apparent value seemed to increase as the spheres employed 

 (glass or ivory) were lighter. (Reich, in Poggendorff's Annalen, Bd. Ixxxv. S. 

 190. Compare also Whitehead Hearn, in the Phil. Trans, for 1847, p. 217 

 229.) The motion of the torsion beam was observed by Baily according to the 

 method of Reich, by means of the image, reflected, as in the magnetic observa- 

 tions of Gauss, in a mirror attached to the middle of the beam. The highly 

 important use of the mirror, which so greatly increases the exactness of the 

 reading, was proposed so long ago as 1826. by Poggendorff (Annalen der Physik, 

 Bd. vii. S. 121). 



( 28 ) p. 33. Laplace, Me'canique celeste, e'd. de 1846, t. v. p. 57. The mean 

 specific weight of granite is estimated at 27 at the most, as the specific weights 

 of the biaxal white mica, and the green uniaxal, are from 2'85 to 3*1, and those 

 of the rest of the constituents of the rock, quartz and felspar, are 2*56 and 

 2'65. Oligoclase itself is only 2'68, and although hornblende may be 3'17, 

 syenite, in which felspar always predominates, still remains much under 2'8. 

 As the weight of clay-slate is 2*69 278, as among limestones it is only in pure 

 dolomites that it attains 2'88, chalk 272, gypsum and rock salt 2'3, I consider 

 the density of the accessible continental crust of the Earth to be nearer 2'6 than 

 2'4. Laplace, on the assumption of the density increasing in arithmetical pro- 



