August 



1900] 



NATURE 



405 



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 Prof. Richarz and Dr. Krigar- 

 Menzel (" Anhang zu den Abhandlungen der Konigl," Preuss. 

 Akad. der Wissenschaften zu Berlin, 1898) 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 aliout 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 i 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 



balance experiment (Richarz and Krigar-Menzel). 



sphere by i^ 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 i 3664 

 mgm., and the mean density of the earth at 5*505. 



This agrees nearly with my own result of 5*49, and it is a 

 curious coincidence that the two most recent balance experi- 

 ments agree very nearly at, say, 5*5 ; and the two most recent 

 Cavendish experiments agree at, say, 5*53. But I confess I 

 think it is merely a coincidence. I have no doubt that the 

 torsion experiment is the more exact, though probably an experi- 

 ment on different lines was worth making. And I am quite 

 content to accept the value of 5 '527 as the standard value for 

 the present. 



And so the latest research has amply verified Newton's cele- 

 brated guess that " the quantity of the whole matter of the 

 earth may be five or six times greater than if it consisted all of 

 water." 



I now turn to another line of gravitational research. When 

 we compare gravitation with other known forces (and those 



NO. 1608. VOL. 62] 



which have been most closely studied are electric and magnetic 

 force's) we are at once led to inquire whether the lines of 

 gravitative force are always straight lines radiating from or to 

 the mass round which they centre, or whether, like electric and 

 magnetic lines of force, they have a preference for some media 

 and a distaste for others. We know, for example, that if a 

 magnetic sphere of iron or cobalt or manganese is placed in a 

 previously straight field, its permeability is greater than the air 

 it replaces, and the lines of force crowd into it, as in Fig. 5. 



Fig. 5. — Paramagnetic sphere placed in a previously straight field. 



The magnetic action is then stronger in the presence of the 

 sphere near the ends of a diameter parallel to the original course 

 of the lines of force, and the lines are deflected. If the sphere 

 be diamagnetic, of water, or copper, or bismuth, the perme- 

 ability being less than that of air, there is an opposite effect, as 

 in Fig. 6, and the field is weakened at the end of a diameter 

 parallel to the lines of force, and again the lines are deflected. 

 Similarly, a dielectric body placed in an electric field gathers in 

 the lines of force, and makes the field where the lines enter and 

 leave stronger than it was before. 



If we enclose a magnet in a hollow box of soft iron placed in 

 a magnetic field, the lines of force are gathered into the iron 

 and largely cleared away from the inside cavity, so that the 

 magnet is screened from external action. 



Now common experience might lead us at once to say that 

 there is no very considerable effect of this kind with gravitation. 

 The evidence of ordinary weighings may, perhaps, be rejected, 

 inasmuch as both sides will be equally affected as the balance 

 is commonly used. But a spring balance should show if there 

 is any large effect when used in different positions above 

 different media, or in different enclosures. And the ordinary 

 balance is used in certain experiments in which one weight is 

 suspended beneath the balance case, and surrounded, perhaps, 

 by a metal case, or perhaps by a water bath. Yet no appreciable 

 variation of weight on that account has yet been noted. Nor 

 does the direction of the vertical change rapidly from place to 

 place, as it would with varying permeability of the ground 

 below. But perhaps the agreement of pendulum results, what- 

 ever the block on which the pendulum is placed, and whatever 

 the case in which it is contained, gives the best evidence that 

 there is no great gathering in, or opening out of the lines of 

 the earth's force by different media. 



Still, a direct experiment on the attraction between two 

 masses with different media interposed was well worthy of trial, 

 and such an experiment has lately been carried out in Ameriqa 



Fig. 6.— Diamagnetic sphere placed in a previously straight field. 



by Messrs. Austin and Thwing (Physical Review, v. 1897, 

 p. 294). The effect to be looked for will be understood from 

 Fig. 7. If a medium more permeable to gravitation is inter- 

 posed between two bodies, the lines of force will move into it 

 from each side, and the gravitative pull on a body, near the 

 interposed medium on the side away from the attractmg body, 

 will be increased. • j r n 



The apparatus they used was a modified kmd of Boys 



