284 Professor John H. Poynting [Feb. 23, 



weight with. 1' between the centres, was about ^ mgm. in a total weight 

 of 20 kgm. or about 1 in 100,000,000. If then a sphere one foot away 

 pulls with 1/10 8 of the earth's pull, the earth being on the average 

 20,000,000 feet away, it is easy to see that the earth's mass is 

 calculable in terms of the mass of the sphere, and its density is 

 at once deduced. The direct aim of this experiment, then, is not 

 G, but the mass of the earth. 



It is not a little surprising that the balance could be made to 

 indicate such a small increase in weight as 1 in 100 million. But 

 not only did it indicate, it measured the increase, with variations 

 usually well within 1% of the double attraction, or to 1 in 5000 

 million of the whole weight, a change in weight which would occur 

 merely if one of the spheres were moved T \> inch nearer the earth's 

 centre. This accuracy is only attained by never lifting the knife 

 edges and planes during an experiment, thus keeping the beam in 

 the same state of strain throughout, and, further, by taking care 

 that none of the mechanism for moving the weights or riders shall 

 be attached in any way to the balance or its case ; two conditions 

 which are absolutely essential if we are to get the best results of 

 which the balance is capable. 



Quite recently another common balance experiment has been 

 brought to a conclusion by Professor Eicharz and Dr. Krigar- 

 Menzel* at Spandau, near Berlin. Their method may be gathered 

 from Fig. 4. A balance of 23 cm., say 9-inch beam, was mounted 

 above a huge lead pile about 2 metres cube, and weighing 100,000 

 kgm. 



Two pans were supported from each end of the beam, one pan 

 above, the other pan below the lead cube, the suspending wires of the 

 lower pans going through narrow vertical tubular holes in the lead. 

 Instead of moving the attracting mass, the attracted mass was moved. 

 Masses of 1 kgm. each were put first, say, one in the upper right-hand 

 pan, the other in the lower left-hand pan, when the pull of the lead 

 block made the right hand heavier and the left hand lighter. Then 

 the weights were changed to the lower right hand and the upper 

 left hand, when the pulls of the lead pile were reversed. When we 

 remember that in my experiment a lowering of the hanging sphere 

 by 1^ inches would give an effect as great as the pull I was measuring, 

 it is evident that here the approach to and removal from the earth 

 by over 2 metres would produce very considerable changes in weight, 

 and, indeed, these changes masked the effect of the attraction of the 

 lead. Preliminary experiments had, therefore, to be made before the 

 lead pile was built up, to find the change in weight due to removal 

 from upper to lower pan, and this change had to be allowed for. The 

 quadruple attraction of the lead pile came out at 1 ■ 3664 mgm., and 

 the mean density of the earth at 5 ■ 505. 



* Annans* zu den Abhandlungen fler Konigl. Preuss. Akad. der Wissen- 

 schaften zu Berlin, 1808. 



