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Mr. J. H. Poynting on a method of [Nov. 21, 



On the Employment of the Balance to determine the Mean Density of the 



Earth. 



In the Cavendish experiment, the attraction of a large sphere of 

 lead of known mass and dimensions upon another smaller sphere also 

 of known mass and dimensions, is measured when the two are an 

 observed distance apart. Comparing this attraction with the weight 

 of the small sphere — that is the attraction of the earth upon it — and 

 knowing the dimensions of the earth, we can deduce the mass of the 

 earth in terms of the mass of the large lead sphere, and so obtain its 

 mean density. The torsion balance, which was invented for the pur- 

 pose by Mitchell, the original contriver of the experiment, has hitherto 

 been used to determine the force exerted by the mass upon the small 

 sphere. In the arrangement here described, I have replaced the 

 torsion balance by the ordinary balance, and have so been able to 

 compare the attraction of a lead sphere with that of the earth upon 

 the same mass somewhat more directly. The results which I have 

 obtained have no value in themselves, but they serve as an example of 

 the employment of the balance for more delicate work than any which 

 it has as yet been supposed able to perform. 



The method is shortly this : — A lead weight (called " the weight ") 

 weighing 452*92 grms. (nearly 1 lb.) hangs down by a fine wire from 

 one arm of a balance, from which the pan has been removed at a 

 distance of about six feet below it, and is accurately counterpoised in 

 the other pan, suspended from the other arm. A large lead mass 

 (called "the mass") weighing 154,2206 grms. (340 lbs.) is then 

 introduced directly under the hanging weight. The attraction of this 

 mass increases the weight slightly and the beam is deflected through 

 an angle which is observed. The value of this deflection in milli- 

 grammes is measured by the employment of riders in the manner 

 described above, and so the attraction of the mass is known. The 

 increase of the weight caused by the mass has been in my experiments 

 about "01 of a milligramme, or l 500 1 0000 tli of the whole weight. 



The balance which I have used is that which I have described 

 above. It was placed in the same room and in the same position as in 

 the weighing experiments. The same method was used to observe the 

 oscillations with a single mirror on the beam. The scale was a simple 

 one etched on glass and not diagonally ruled. It had about 50 

 divisions to the inch, and the numbers increased from above down- 

 wards, so that an increase in the weight hanging from the left arm 

 was indicated by a lower number on the scale. 



The weight which is suspended by a very fine brass wire from the left 

 arm, passing through a hole in the bottom of the balance case, hangs 

 in a double tin tube, 4 inches in diameter, to protect it from air 

 currents. At the botttom of the tube is a window, through which can 



