318 



Major J. Herschel. On Gravimeter s. [Jan. 20, 



A very similar, if not identical, design having been published in the 

 " Comptes Rendus" of the Paris Academy, by the late M. Babinet, the 

 author contributes to the history of the subject some letters written 

 by Babinet anterior to the said publication, with the object of showing 

 the concurrent identity and independence of the two designs. 



The principle on which the bathometer of Dr. Siemens appears to 

 be constructed is then considered. Although intended to measure 

 sea-depths, and on that account called a bathometer, it is none the 

 less a gravimeter in principle, and its construction, when properly 

 understood, reminds one forcibly of the typical form as originally 

 proposed, by way of illustration, by Sir John Herschel in the " Out- 

 lines of Astronomy." The indications of the bathometer, it should be 

 said, are only to be interpreted empirically. 



The principle on which the gravimeters of Broun and Babinet are 

 designed is thus emphasized by the latter : — " Ce qui distingue surtout 

 ma methode, c'est le fractionnement de la pesanteur, de maniere a 

 equilibrer l'effet d'un poids considerable, par la torsion d'un fil me- 

 tallique de force moyenne, et restant dans les limites de l'elasticite 

 parfaite." 



It is doubtful whether Babinet's conception ever took practical 

 form as an actually constructed instrument ; at any rate, no published 

 record of its performance exists. Broun's design, on the other hand, 

 after many vicissitudes, took the shape of the instrument now under 

 consideration. It was made for him by Miiller of Stuttgart, who 

 appears to have taken extraordinary pains to give effect to the in- 

 tentions of the designer. 



Mr. Broun exhibited his gravimeter in the Loan Collection of 

 Scientific Instruments at South Kensington, in 1876, where it was 

 placed alongside of Dr. Siemens' bathometer. A description of it 

 appeared in the catalogue, corrected by himself, but it is faulty and in- 

 adequate, owing to the declining health of the owner having prevented 

 his acquiring a practical acquaintance with his instrument; nor was 

 he able to do so up to the time of his death. It was, therefore, with 

 the very scantiest preparation that the present examination was 

 undertaken. 



The principle of Broun's gravimeter is easily understood, but the 

 manipulation of the instrument is so troublesome that it is more than 

 doubtful whether in its present form results of any practical value 

 could be obtained with it. The various grounds for this opinion are 

 explained in the course of the report. As a fact, no observations, such 

 as were naturally desired to test its sensitiveness, could by any means 

 be obtained, owing to a radical defect of construction, which the 

 maker has unfortunately failed in overcoming, although special atten- 

 tion has been bestowed upon the point. To explain this, some 

 description of the instrument is unavoidable. 



