Reviews — Prof. Poynting — Mean Density of the Earth. 177 



in a cellar at the Mason College, Birmingham, its foundation having 

 no connection with the brick tloor of the cellar. The turn-table, 

 on which the attracting mass M rests, has also an isolated foundation 

 separate from the former. The masses A and B each weigh about 

 47^ lbs. and the mass J/ about 338 lbs., all three being composed of 

 an alloy of lead and antimony of specific gravity 10-4. The distance 

 between the centres of M. and yl or B is about 12^ inches in the 

 lower position of the latter, and about 24^ inches in their higher 

 position. 



To magnify the tilting of the balance, a double-suspension mirror 

 was used, similar to that emploj^ed by Messrs. G. H. and H. Darwin 

 in their experiments on the lunar disturbance of gravity. The 

 mirror is suspended by two eye-holes on a fine silk thread, one end 

 of which is attached to a small bracket projecting from the lower 

 end of the pointer of the balance, the other to a fixed bracket close 

 to the former, so that the plane of the mirror is perpendicular to 

 that of the balance. The distance between the points of support 

 of the silk thread being 2^ of an inch, and the length of the balance- 

 pointer 2 feet, it follows that the mirror turns through an angle 

 150 times as great as the balance-beam. 



In the room over the cellar and close to a hole in the floor are 

 the telescope, provided with cross-wires, and a scale illuminated by 

 a lamp. The light from the scale is reflected by an inclined mirror 

 in the cellar so as to meet the double-suspension mirror horizontally, 

 is reflected by it and by the inclined mirror again, and returns to 

 the telescope. If the balance-beam turns through a very small 

 angle, the image of the scale seen in the telescope is shifted over 

 the cross-wires. Two small riders, each weighing one centigramme, 

 are arranged so that their points of support are on the balance-beam 

 at a distance of one inch on either side of the knife-edge. One 

 being raised from the beam, the other is lowered on to it, and the 

 number of divisions of the scale corresponding to the deflection of 

 the beam so produced is compared with the number of divisions 

 corresponding to the deflection produced by the attraction of M 

 being taken away from A and added to B. This ratio gives the 

 increase in the weight of A or B due to the attraction of M when 

 vertically below it. 



It will be unnecessary here to enter further into the details of 

 the experiment, or to describe the numerous precautions taken to 

 ensure accuracy. The chief disadvantage of the method is that 

 " the disturbances due to air-currents are greatest in the vertical 

 direction, that of the displacement to be measui'ed," and the larger 

 the apparatus the greater are the errors produced by them. If the 

 experiment were to be repeated, the author would make the apparatus 

 small, for it could then be kept at a more uniform temperature and 

 it would be more handy to adjust. " At the same time," he con- 

 cludes in his characteristic manner, "it is only fair to say, on behalf 

 of the large apparatus, that some errors have been magnified on a 

 like scale till they have become observable, and so could be inves- 

 tigated and eliminated. Starting with a small apparatus they would 



DECADE IV. — VOL I. — NO. IV. 12 



