526 



Major J. Herschel. 



We must look for accuracy which may be represented by + 0*1 

 second, or the instrument is not worth testing severely. This corre- 

 sponds to 0*35 millim. of glycerine, or 35 divisions of the micrometer. 



The foregoing description and investigation represent what I had 

 learnt about the gravimeter by inspection, measurement, calculation, 

 and reflection, before meeting with any instruction from the designer. 

 In his letter to General Walker, Mr. Broun mentions having seen and 

 corrected the proof of the description which was to appear in a new 

 edition of the Catalogue of Instruments exhibited in the Special 

 Loan Collection. Not expecting that a catalogue description would 

 be otherwise than brief, and confined to principal features such as I 

 could not fail to perceive unaided, I did not wait until I could procure 

 it, to study the instrument. Some delay also occurred before I 

 received, through the courtesy of the Secretary of the Science and 

 Art Department, the extract in question. It seems right to mention 

 this, in explanation of the independence which will be remarked in 

 what I have said. Whereas, in what follows, I acknowledge the said 

 description as an additional source of information. Mr. Broun's 

 description is accordingly inserted here ; and I shall then add some 

 comments upon points which seem to need further elucidation or 

 notice. 



" 421d. Gravimeter. An instrument for the measurement of 

 the variations of the earth's attractive force, invented by J. A. 

 Broun, F.R.S., and constructed from his drawings by Dr. C. 

 Miiller, of Stuttgart. /. Allan Broun, F.B.8. 



" The instrument consists of a weight suspended by two gold 

 wires ; a single wire fixed to the top of the weight and passing 

 through its centre carries a cylindrical lever; when the lever is 

 turned through 360° at the normal (say southern) station, the 

 torsion of the single wire thus produced carries the weight round 

 through an angle of 90°. The forces then in equilibrium are, 

 the torsion force of the single wire and the attraction of the 

 earth on the weight, which, as the two wires are no longer vertical, 

 has been slightly raised and seeks to attain its lowest point. 



" On proceeding from a southern to a more northerly station 

 the earth's attraction increases ; the amount of this increase may 

 be measured in two ways : — 



"1st. The lever will require to be turned through more than 

 360° in order to carry the weight to the height due to turning 

 it through 90°. (Had the station been more southerly the lever 

 would be turned through less than 360°.) The difference of 

 the angle from 360° measures the increase (or diminution) of 

 weight. 



