On Gravimeters. 



515 



I have endeavoured to ascertain, by enquiry from a gentleman who 

 was associated with Babinet in physical researches of this nature, 

 whether the instrument was ever constructed ; but I do not gather 

 from the reply that such was the case. It would seem as if there was 

 too much uncertainty as to the constancy of the force relied upon in 

 torsion. 



The original paper communicated by Broun to the Royal Society of 

 Edinburgh follows immediately upon two others by the same author, 

 in one of which the theory of the bifilar magnetometer is considered, 

 and mention is there made of the elasticity of the suspending wires. 

 Their effect is considered to be of little importance. The effect of 

 temperature in modifying the elasticity is also alluded to. "We cannot 

 therefore suppose Mr. Broun to have disregarded these considerations 

 altogether. It is perhaps the more remarkable that he has left no 

 indication of being alive to the really interesting balance which 

 occurs in his instrument, and which must equally have occurred, and 

 can hardly have escaped his notice, in the former one. 



In his letter, above referred to, he speaks of his gravimeter as if 

 the instrument ultimately perfected was substantially the same as the 

 one described in 1861. But, as I have had occasion to point out, it 

 differs in one respect which I cannot but consider important — the 

 difference between a spiral balance spring and a twisted wire. It is 

 to be regretted that we are not in possession of any information as to 

 the reasons which induced him to discard the one in favour of the 

 other. In any case, the change is distinctly a part of the history of 

 the invention. The next step was the construction of the instrument 

 as mentioned in his letter ; and then its exhibition and description by 

 himself in the Catalogue of the Loan Collection of Scientific Instru- 

 ments at the South Kensington Museum, where, he tells us sugges- 

 tively, it was shown alongside of Mr. Siemens' bathometer. We 

 may now turn our attention to this last. 



My knowledge of the bathometer designed by Dr. C. W. Siemens 

 is confined to the account given of it in the " Philosophical Trans- 

 actions " for 1876, and I should not presume to examine it were it 

 not necessary to do so for the purpose of this review. Dr. Siemens, 

 it is true, regards it, in the introductory passages of that paper, as 

 having a different sphere of action ; but, on a careful consideration of 

 the principles — so far as I understand them — of its construction, it 

 appears to me to belong decidedly to the class of instrument, whatever 

 we may call them, typified by the one already mentioned as described 

 in the " Outlines of Astronomy." The special purposes for which iu 

 has been designated, and the actual uses to which it has been put, do 

 not of themselves preclude it from being set to the more general pur- 

 poses and uses of a gravimeter. 



Reference is made in the cited paper to another form of instrument 



