388 PHILOSOPHICAL TRANSACTIONS. [ANNO 17Q8. 



a dirty olive colour. The colour of the contents of the phials c and d was changed 

 nearly in the same manner ; and having been suffered to stand quiet 2 or 3 days, 

 to settle, the solution was found to be quite colourless, and the sediment to be 

 deeply coloured. There was however a very remarkable difference in the hues of 

 the 2 phials ; that of the phial c being of a light greenish-blue ; while that in the 

 phial d was indigo, and of so deep a tint, that it might easily have been taken 

 for black. 



These appearances were certainly very striking, and well calculated to excite my 

 curiosity ; but I am so much engaged in public business, that it is not at present 

 in my power to pursue these inquiries farther. I wish that what I have done may 

 induce others, who have more time to spare, to devote some portion of their leisure 

 to these interesting investigations. 



XXI. To Determine the Density of the Earth. By H. Cavendish, Esq., F. R. S. 9 



andJ.S. p. 46g. 



Many years ago, the late Rev. John Michell, of this Society, contrived a me- 

 thod of determining the density of the earth, by rendering sensible the attraction 

 of small quantities of matter; but, as he was engaged in other pursuits, he did 

 not complete the apparatus till a short time before his death, and did not live to 

 make any experiments with it. After his death, the apparatus came to the Rev. 

 Francis John Hyde Wollaston, Jacksonian Professor at Cambridge, who, not 

 having conveniences for making experiments with it, in the manner he could wish, 

 was so good as to give it to me. The apparatus is very simple; it consists of a 

 wooden arm, 6 feet long, made so as to unite great strength with little weight. 

 This arm is suspended in an horizontal position, by a slender wire 40 inches long, 

 and to each extremity is hung a leaden ball, about 2 inches in diameter; and the 

 whole is inclosed in a narrow wooden case, to defend it from the wind. 



As no more force is required to make this arm turn round on its centre, than 

 what is necessary to twist the suspending wire, it is plain, that if the wire is suffi- 

 ciently slender, the most minute force, such as the attraction of a leaden weight a 

 few inches in diameter,, will be sufficient to draw the arm sensibly aside. The 

 weights which Mr. Michell intended to use, were 8 inches diameter. One of 

 these was to be placed on one side the case, opposite to one of the balls, and as 

 near it as could conveniently be done, and the other on the other side, opposite to 

 the other ball, so that the attraction of both these weights would conspire in draw- 

 ing the arm aside ; and when its position, as affected by these weights, was ascer- 

 tained, the weights were to be removed to the other side of the case, so as to 

 draw the arm the contrary way, and the position of the arm was to be again deter- 

 mined; consequently, half the difference of these positions would show how much 

 the arm was drawn aside by the attraction of the weights. In order to determine 

 from hence the density of the earth, it is necessary to ascertain what force is re- 

 quired to draw the arm aside through a given space. This Mr. Michell intended 



