On Gravimeters. 



537 



gravity in opposing the torsion of the single wire. If Mr. Broun was 

 aware of this, it is strange that he gives no hint of it in mentioning 

 the forces in equilibrium ; anyhow, the third force is there, and might, 

 I think, be taken advantage of. I found by trial that the torsion of 

 the single wire might be increased so cautiously as to cause the major 

 weight to stand nearly stationary at the position of unstable equili- 

 brium. If the angle of position of the lower weight were read off (by 

 vernier or otherwise) when this happened, both right hand and left 

 hand, I imagine a delicate measure of the variation of gravity would 

 be obtained, without the need of any appliances for varying the 

 weight, or of a collimator. It would be foreign to the purpose of this 

 paper to pursue this design further here ; it is enough to have indi- 

 cated it in connexion with the principle involved in Broun's design. I 

 will only add that the two principles here indicated, viz., the balancing 

 of gravity by torsion, and the determination of the condition of equili- 

 brium when unstable, are both involved in a simple bifilar suspension ; 

 -and I see reason to think that this form of gravimeter, from its ex- 

 treme simplicity and great adaptability, is worth consideration.* 



It now only remains to offer some apology for the length of this 

 report, and for its discursiveness. It will have been quite apparent to 

 any one who has had the patience to read it, that it has been written 

 from day to day, as the examination proceeded ; and now that the 

 latter has to be closed without anything of the nature of an " observa- 

 tion " having been possible, it may be that there is a certain advan- 

 tage in letting the facts, so brought forward, tell their own tale. 

 Much, no doubt, could well be spared, but at the risk of laming the 



narrative At the same time, I may say that 



I am quite willing to renew the attack, and do what can be done to 

 reach a more promising conclusion, and to make the instrument 

 efficient ; provided I am so instructed. In that case, however, the 

 sanction of the Royal Society, to whom it belongs, will be also 



* If (P+j?) — sin0=2p£ be the general statical equation of a simple bifilar 

 E> 



system, the equilibrium is unstable when sec = — ? + P . in which position 



2p R 



(po= — tan O . From this it is at once apparent, since <p must be positive, that 9 

 must lie between 90° and 180°. It must also be greater than 90°, or <p would be in- 

 finite ; but beyond this I do not see any theoretical restriction. I think it would 

 be advisable to make O as large — several revolutions — as the wires will bear without 

 injury. All that is necessary is that they be provided with means of turning their 

 upper ends, until the weight reaches its position of unstable equilibrium (which 

 should happen when is little more than 90°), and with means of recording the 

 angle through which they have been turned so as to reach this condition. The ap- 

 plicability of such an arrangement, as a sensitive gravimeter, will depend entirely on 

 the adaptation ; which, I believe, would be found quite feasible. The equations 

 here given are sufficient to determine suitable proportions. 



VOL. XXXII. 2 Q 



