446 Defcr'iptton of an Apparatus fot 



of time a portion of oxygen having been carried off by unknown caufes, converted tliem t# 

 the ftate of oxide, and changed the red colour to green. 



From paragraph III. it follows that the Siberian red lead contains 



By aoalyCs. Experiment V, Byfynthefis, EiperimentVI. 

 Oxide of lead - - 63,96 65,12 



Chromic acid - - 36,40 34}88 



Exper'tmsnts to determine the Denfty of the Earth. By HenRT CAFENDISHy Efp 



F.R.S. andA.S* 



M. 



. A N Y years ago the Rev. John Michell, F. R. S. contrived a method of determining 

 the denfity of the earth, by rendering fenfible the attra£llon of fmall quantities of mat- 

 ter J but, as he was engaged in other purfuits, he did not complete the apparatus till * 

 Ihort time before his death, and did not live to make any experiments with it. After hi» 

 death the apparatus came to the Rev. Francis John Hyde Wollafton, Jackfonian profeflbr 

 at Cambridge, who, not having conveniences for making experiments with it in the man- 

 ner he could wifh, prefented it to Mr. Cavendifh. 



The apparatus is very (imple : it confifts of a wooden arm fix feet long, made fo as to- 

 unite great ftrength with little weight. This arm is fufpended in an horizontal pofition by 

 a flendcr wire forty inches long, and to each extremity is hung a leaden ball about two 

 inches in diameter ; and the whole is inclofed in a narrow wooden cafe, to defend it from 

 the wind. 



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

 to twift the fufpending wire, it is plain that, if the wire is fufficiently flender, the mod mi- 

 nute force, fuch as the attra£lion of a leaden weight a few inches in diameter, will be 

 fufficient to draw the arm fenfibly afide. The weights which Mr. Michell intended to- 

 ufe were eight inches diameter. One of thefe was to be placed on one fide of the cafe,, 

 oppofite to- one of the balls, and as near it as could conveniently be done, and the other on 

 the other fide, oppofite to the other ball, fo that the attradlion of both thefe weights would 

 confpire in drawing the arm afide ; and when its pofition, as affeded by thefe weights, was 

 afcertained, the weights were to be removed to the other fide of the cafe, fo as to draw the 

 arm the contrary way, and the pofition of the arm was to be again determined ; and confe- 

 quently half the difference of thefe pofitions would fhew how much the arm was drawn.- 

 afide by the attraflion of the weights. 



In order to determine from hence the denfity of the earth, it is neceffary to afcertain: 

 what force is required to draw the arm afide through a given fpace. This Mr. Michell 

 intended to do by putting the arm in motion, and obferving the time of its vibrations, from, 

 which it may be eafily computed-f-. ' 



• Abridgment of his paper in the Philofophical Tranfaftions^ 1798, p. 469. 



t Mr. Coulomb has in a variety of cafes ufed a contrivance of this kind for trying fmall attraftions ; but Mr. 

 Michell informed Mr. C. of his intention of making this experiment, apd of the method he intended to ufe, be. 

 fere the publication of acy of Mr. Coulomb's experiments. 



6. Mr; 



