On Gravimeters. 



507 



" On Gravimeters ; with special reference to a Torsion Gra^i- 

 meter, designed by the late J. Allan Broun, F.R.S." By 

 Major J. Herschel, R.E., F.R.S., Deputy Superintendent of 

 the Survey of India. Received October 31, 1880. Read 

 January 20, 1881. 



[Plates 8, 9.] 



The present paper consists mainly of two parts, of which the second 

 is the earlier, the first having been afterwards prepared as a historical 

 introduction. 



In August last a letter was addressed by Major- General J. T. Walker, 

 R.E., C.B., F.R.S., Surveyor-General of India, to the Director- General 

 of Stores for the Indian Government, enclosing copy of a letter from 

 the late Mr. Broun on the subject of his gravimeter. The object of 

 General Walker's letter was to trace, if possible, the instrument in 

 question, and to secure it for the Indian Government. It was also 

 suggested that I might be able to judge of its efficiency. I also received 

 a communication to the same effect, and in consequence offered my 

 services. 



On enquiry it appeared that the gravimeter had become the property 

 of the Royal Society. The loan of it was obtained and I was requested 

 to call and see it. It was then arranged that I should take the instru- 

 ment and report upon its capabilities at greater leisure than was 

 possible at the India Office. 



The gravimeter is an instrument obviously requiring very careful 

 manipulation, and as I was not for some time after it came into my 

 hands in possession of any description of its intended use, the exami- 

 nation was attended with anxiety as well as difiiculty. It occupied 

 me for three weeks, during which time I was in constant fear lest some 

 awkwardness should prevent my returning it uninjured. I purposely, 

 therefore, wrote what I learnt about it from day to day ; and this will 

 perhaps excuse the length of my report, which was eventually sub- 

 mitted to the authority from whom I received the instrument. 



The course of the examination necessarily led to my becoming 

 acquainted with the subject in several ways, but the narrative form 

 and the immediate object of the report precluded any discussion. 

 There appeared nevertheless to be sufficient in the study of the instru- 

 ment itself, and especially of its dimensions and the weights of its 

 parts, to justify a desire for publication. I accordingly solicited per- 

 mission to communicate the report to the Royal Society, the owner of 

 the instrument. This having been granted, it then seemed permissible 

 to preface the report by a sketch of some of the circumstances bearing 

 on the question of priority of invention, and other matters in con- 



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