On Gr ammeters. 



509 



spring. These evidently are the points towards which later improve- 

 ments may be expected to be directed. 



The elasticity of such a coiled spring partakes, I believe, of two 

 characters. It would depend very much upon the pitch of the coil 

 whether the elasticity would be chiefly that of bending or of torsion. 

 If, as I suppose, these are not the same, there is to some extent a 

 change of principle in leaving the coil, and adopting the straight sus- 

 pending wire, the resistance to torsion of which is substituted (in the 

 next form) for the resistance to extension of the coil as above. With 

 a view to meeting one of the two difficulties mentioned, it might 

 become a question whether the required constancy would be more 

 attainable in the one direction or in the other. The modifications of 

 design which actually occurred, however, do not appear to have arisen 

 from considerations of this nature; but were aimed at securing greater 

 facility, constancy of resistance being assumed. 



About the year 1860 two inventors devoted their attention indepen- 

 dently to methods of statical gravimetry. In each case we have 

 means of perceiving the growth and modification of their ideas, and 

 it must be regarded as a curious coincidence that they arrived at a 

 nearly identical result. One of them eventually designed, and caused 

 to be constructed, an instrument which is the occasion of this paper. 

 The other, so far as I have been able to learn, did not give practical 

 effect to his conceptions in the shape of a finished instrument ; at 

 least he has not published any account of observations. 



The remarkable similarity which undoubtedly exists between the 

 gravimeter as made under Mr. Broun's instructions and that which 

 was proposed by M. Babinet, has naturally raised the question of 

 priority. In a letter written by the former to General Walker, 

 Mr. Broun expresses himself in a way to show that he can with diffi- 

 culty divest himself of the idea that M. Babinet had been led to his 

 design by more or less indirect cognizance of what Mr. Broun had 

 published or communicated orally to mutual acquaintances. As it 

 happens that I have access to correspondence bearing on the subject, 

 1 have looked into it somewhat closely, and think it not undesirable 

 to publish some of the letters written by M. Babinet to my father at 

 the time when he was busied about it. 



The earliest publication on the subject occurs in the " Proceedings 

 of the Royal Society of Edinburgh" lor 1861, in which (pp. 411-412, 

 February 4) Broun describes a machine, in course of construction, 

 for the purpose of measuring small changes of gravity by the 

 balancing of a bifilar disturbance by a hair spring placed below the 

 suspended weight. 



In February, 1863, appeared Babinet's design,* in which a similar 



* " Comptes Rendus," hi, 1863, pp. 244-248 ; and " Cosmos," 1863, pp. L77-181. 



