RECENT STUDIES IN GRAVITATION. 



205 



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 l)e made to indicate 

 such a small increase in weight as 1 in 100,000,000. But not only did 

 it indicate, it measured the increase, with variations usually well within 

 1 per cent of the double attraction, or to 1 in 5,000,000,000 of the 

 whole weight, a change in weight which would occur merely if 

 one of the spheres were moved one-fortieth inch nearer the earth's 

 center. 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, b}' taking care that none of 

 the mechanism for moving the weights or riders shall be attached in 



ii^ 



Fig. 4.— Common balance experiment (Richarz and Krigar-Menzel). 



any way to the balance or its case; two conditions which are abso- 

 lutely essentialTif we are to get the best results of which the balance 

 is capable. 



Quite recently anothci* common balance experiment has been Ijrought 

 to a conclusion by Professor Richarz and Dr. Krigar-Menzel '' at Span- 

 dau, near Berlin. Their method may be gathered from tig. 4. A 

 balance of 23 cm., say D-inch ))eam, was mounted above a huge lead 

 pile about 2 meters cube, and weighing 100,000 kilograms. 



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

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

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



* Aiiliang zu den Abhandlungeu der Konigl. 

 zu Berlin, 1898. 



Freuss. Akad. der Wissenschaften 



