M. Reich on the mean Demity of the Earth, 283 



of this observation, in Delambre, Histoire de VAstronomie Ancienne, 

 torn. i. p. 350, et seq. 



On the Repetition of the Cavendish Experiment, for determining 

 the Mean Density of the Earth. By the President. 



It is well known to most of the Members of the Society, that the 

 Council has long had it in comtemplation to repeat the celebrated 

 and interesting experiment of the late Mr. Cavendish for determining 

 the Mean Density of the Earth, and that a Committee was appointed 

 more than two years ago to consider of its practicability. The object 

 is now in a fair way of being accomplished. Her Majesty's govern- 

 ment having been jileased to "grant the sum of 500^. towards defray- 

 ing the requisite expenses. The apparatus is, at this moment, in 

 the course of being erected, and as soon as it is completed, the ex- 

 periments will be commenced. 



During the time, however, that the subject has been in agitation 

 in this country, it appears that the same experiment has been un- 

 dertaken by M. F. Reich, Professor of Natural Philosophy in the 

 Academie des Mines, at Freyberg, in Saxony. The details of the 

 experiment are given in a memoir, read by him at the German Sci- 

 entific Association, which met at Prague in the month of September 

 last. Whether this memoir has yet been published, the Author has 

 been unable to ascertain ; but an abstract of it has appeared in some 

 of the foreign journals, from which the following particulars are 

 collected. 



The method followed by M. Reich appears to have been exactly 

 the same as that of Cavendish. The apparatus was erected in a 

 large room under the buildings of the Academie, the windows of 

 which were carefully closed up, and other precautions taken to pre- 

 serve a uniform temperature. The torsion-balance, carrying two 

 small leaden balls at the extremities of its arms, was encased in 

 a wooden box, of dimensions just sufficient to allow room for the 

 oscillations. To avoid currents of air, the oscillations were ob- 

 served by means of a ^telescope fixed outside the door of the room 

 in which the apparatus was placed, and directed on a mirror at- 

 tached to the extremity of the arm, and illuminated by a lamp, also 

 placed outside the room. The masses, whose attraction was to be 

 measured, were spheres of lead, weighing 45 kilogrammes, or 

 695,061 grains. They were suspended by brass rods to a beam, 

 moveable about a vertical axis ; and which by means of cords and 

 puUies, the observer, without entering the room, could bring into 

 any required position, with reference to the direction of the arms 

 of the torsion-balance. It was found, however, most convenient, 

 to use only one of the spheres. The principal correction required 

 is for the moment of the arm of the balance. This was computed 

 by a method similar to that which was employed by Gauss for de- 

 termining the moment of inertia of his magnetic bars. 



Nearly two years were consumed in the necessary preparations ; 

 but when completed, M. Reich was enabled to perform the experi- 

 ments during the three months of June, July and August, 1837. 

 Each observation required the determination of three quantities :— 



