34 COSMOS. 



feet and at the level of the sea (as, for instance, Carlini's 

 observations at the Hospice of Mont Cenis, and Biot and 

 Mathieu's at Bordeaux) ; and, lastly, to the delicate and 

 thoroughly decisive experiments undertaken in 1837 by 

 Reich and Bailey with the ingeniously constructed torsion- 

 balance which was invented by John Mitchell, and subse- 

 quently given to Cavendish by Wollaston.* The three 

 modes of determining the density of our planet (by vicinity 

 to a mountain mass, elevation of a mountainous plateau, 

 and the balance) have already been so circumstantially de- 

 tailed in a former part of the Cosmos (vol. i., p. 157), that it 

 only remains for us to notice the experiments given in 

 Reich's new treatise, and prosecuted by that indefatigable 

 observer during the interval between the years 1847 and 

 1850.t The whole may, in accordance with the present 

 state of our knowledge, be arranged in the following man- 



Shehallien, according to the mean of the maximum 4:"867 and 



the minimum 4*559, as found by Playfair 4-713 



Mont Cenis, observations of Carlini, with the correction of 



Giulio 4-950 



* Baily, Exper. with the Torsion Rod for determining the mean Density 

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

 p. 24. 



t Keich, Neue Versuche mit der Drehwage, in the Ahhandl. der ma- 

 thern. physischen Classe der Kbn. Sdchsischen Gesellschaft der Wissen- 

 schaften zu Leipzig, 1852, bd. i., s. 405, 418. The most recent experi- 

 ments of my respected friend Professor Reich approximate somewhat 

 more closely to the results given in Baily's admirable work. I have 

 obtained the mean 5-5772 from the whole series of experiments : (a) 

 with the tin ball and the longer thicker copper wire, the result was 

 5-5712, with a probable error of 0-0113 ; (b) with the tin ball, and with 

 the shorter thinner copper wire, as well as with the tin ball and the 

 bi-filar iron Avire, 5-5832, with a probable error of 0-0149. Taking 

 this error into account, the mean in (a) and (b) is 5-5756. The re- 

 sult obtained by Baily, and which was certainly deduced from a larger 

 number of experiments (5-660), might indeed give us a somewhat 

 higher density, as it obviously rose in proportion to the greater light- 

 ness of the balls that were xised in the experiments, which were either 

 of glass or ivory. (Reich, in Poggend., Annalen, bd. Ixxxv., s. 190. 

 Compare also Whitehead Hearn, in the PhUos. Transact, for 1847, p. 

 217-229.) The motion of the torsion-balance was observed by Baily 

 by means of the reflection of a scale obtained from a mirror, which 

 was attached to the middle of the balance, a method that had been 

 first suggested by Reich, and was employed by Gauss in his magnetic 

 observations. The use of such a mirror, which is of great importance, 

 from the exactness with which the scale may be read off, was proposed 

 by PoggendoriF as early as the year 1826. {Annalen der Physik., bd. 

 rii., s. 121.) 



