4c6 



NA TURE 



[August 23, igoo 



apparatus (Fig 8). Two small gold masses in the form of 

 short vertical wires, each '4 gm. in weight, were arranged at 

 different levels at the ends virtually of a torsion rod 8 mm. long. 

 The attracting masses M^ Mj were lead, each about i kgm. 

 These were first in the positions shown by black lines in the 

 figure, and were then moved into the positions shown by 

 dotted lines. The attraction was measured first when merely 

 the air and the case of the instrument intervened, and then 

 when various slabs, each 3 cm. thick, 10 cm. wide and 29 cm. 

 high, were interposed. With screens of lead, zinc, mercury, 

 water, alcohol or glycerine, the change in attraction was at the 

 most about i in 500, and this did not exceed the errors of 

 experiment. That is, they found no evidence of a change in 

 pull with change of medium. If such a change exists, it is not 

 of the order of the change of electric pull with change of 

 medium, but something far smaller. Perhaps it still remains 

 just possible that there are variations of gravitation perme- 

 ability comparabfe with the variations of magnetic permeability 

 in media such as water and alcohol. 



Yet another kind of effect might be suspected. In most 

 crystalline substances the physical properties are different along 

 different directions in a crystal. They expand differently, they 

 conduct heat differently, and they transmit light at dififerent 

 speeds in different directions. We might, then, imagine that 

 the lines of gravitative force spread out from, say, a crystal 

 sphere unequally in different directions. Some years ago, Dr. 

 Mackenzie {Physical Review, ii. 1895, p. 321) made an 

 experiment in America in which he sought for direct 

 evidence of such unequal distribution of the lines of force. He 

 used a form of apparatus like that of Prof. Boys (Fig. 2), the 

 attracting masses being calc spar spheres about 2 inches in 



medium in radia 



field of force. 



diameter. The attracted masses in one experiment were small 

 lead spheres about h, gm. each, and he measured the attraction 

 between the crystals and the lead when the axis of the crystals 

 were set in various positions. But the variation in the attraction 

 was merely of the order of error of experiment. In another 

 experiment the attracted masses were small calc spar crystal 

 cylinders weighing a little more than ^ gm. each. But again 

 there was no evidence of variation in the attraction with 

 variation of axial direction. 



Practically the same problem was attacked in a different way 

 by Mr. Gray and myself {Phil. Trans., 192, 1899, A, p. 245). 

 We tried to find whether a quartz crystal sphere had any 

 directive action on another quartz crystal sphere close to it, 

 whether they tended to set with their axes parallel or crossed. 



It may easily be seen that this is the same problem by con- 

 sidering what must happen if there is any difference in the 

 attraction between two such spheres when their axes are parallel 

 and when they are crossed. Suppose, for example, that the 

 attraction is always greater when their axes are parallel, and 

 this seems a reasonable supposition, inasmuch as in straight- 

 forward crystallisation successive parts of the crystal are added 

 to the existing crystal, all with their axes parallel. Begin, 

 then, with two quartz crystal spheres near each other with their 

 axes in the same plane, but perpendicular to each other. 

 Remove one to a very great distance, doing work against their 

 , mutual attractions. Then, when it is quite out of range of 

 appreciable action, turn it round till its axis is parallel to that of 

 the fixed crystal. This absorbs no work if done slowly. Then 

 let it return. The force on the return journey at every point is 

 greater than the force on the outgoing journey, and more work 

 will be got out than was put in. When the sphere is in its first 

 position, turn it round till the axes are again at right angles. 



Then work must be done on turning it through this right angle 

 to supply the difference between the outgoing and incoming 

 works. For if no work were done in the turning, we could go 

 through cycle after cycle, always getting a balance of energy 

 over, and this would, I think, imply either a cooling of the 

 crystals or a diminution in their weight, neither supposition 

 being admissible. We are led, then, to say that if the 

 attraction with parallel axes exceeds that with crossed axes, 

 there must be a directive action resisting the turn from the 

 crossed to the parallel positions. And conversely, a directive 

 action implies axial variation in gravitation. 



The straightforward mode of testing the existence of this 

 directive action would consist in hanging up one sphere by a 

 wire or thread, and turning the other round into various 

 positions, and observing whether the hanging sphere tended to 

 twist out of position. But the action, if it exists, is so minute, 

 and the disturbances due to air currents are so great, that it 

 would be extremely difficult to observe its effect directly. It 

 occurred to us that we might call in the aid of the principle of 

 forced oscillations, by turning one sphere round and round at a 

 constant rate, sO that the couple would act first in one direction 

 and then in the other, alternately, and so set the hanging sphere 

 vibrating to and fro. The nearer the complete time of vibration 

 of the applied couple to the natural lime of vibration of the 



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NO. l6o8. VOL. 62] 



Fig. 8. — Experiment on gravitative permeability (Austin and Thwing). 



hanging sphere, the greater would be the vibration set up. 

 This is well illustrated by moving the point of suspension of a 

 pendulum to and fro in gradually decreasing periods, when the 

 swing gets longer and longer, till the period is that of the 

 pendulum, and then decreases again. Or by the experiment of 

 varying the length of a jar resounding to a given fork, when 

 the sound suddenly swells out as the length becomes that which 

 would naturally give the same note as the fork. Now, in looking 

 for the couple between the crystals, there are two possible cases. 

 The most likely is that in which the couple acts in one way 

 while the turning sphere is moving from parallel to crossed, and 

 in the opposite way during the next quarter turn from crossed to 

 parallel.. That is, the couple vanishes four times during the 

 revolution, and this we may term a quadrantal couple. But it 

 is just possible that a quartz crystal has two ends like a magnet, 

 and that like poles tend to like directions. Then the couple 

 will vanish only twice in a revolution, and may be termed a 

 semicircular couple. We looked for both, but it is enough now 

 to consider the possibility of the quadrantal couple only. 



Our mode of working will be seen from Fig. 9. The hanging 

 sphere, '9 cm. in diameter and 1 gm. in weight, was placed in a 

 light aluminium wire cage with a mirror on it, and suspended by 

 a long quartz fibre in a brass case with a window in it opposite 

 the mirror, and surrounded by a double-walled tinfoiled wood 



