On Gravimeters. 



53V> 



senting (each within 1 gr., I think) the major and minor weights, as 

 they actually hang : they weigh (according to my scale weights) 

 3,100 and 624 grs. respectively. The sum agrees with the second 

 weighing to a grain — the exact agreement being unintentional. 



These weighings do not include the platinum wire riders. 



I may add here an account of the measurement of the glass 

 plunger — described by Mr. Broun as " calibrated " to 1 millim. 

 My rough (?) measurement had indicated, as above said, "048 inch as 

 its diameter. The cathetometer microscope seemed to show it about 

 120 divisions, or 1*2 millims. = '049 inch, but I did not trust this. I 

 hung a small weight by a fibre of raw silk and wound up 100 turns 

 of it on the glass rod. The length absorbed was 14'9 inches. I esti- 

 mate the diameter of the silk, which was coarse, at '001 ; but it is 



0*149 



very uneven. This would give — , 001 = *046 inch. Not satis- 



fied, I repeated this with some fine silver wire, annealing it first. 

 This gave similarly ^-i?^— '0036 = "0454 inch. The diameter of the 



7T 



wire was got by measuring the length covered by the close coil of 

 100 turns. The true diameter of the glass rod is rather larger than 

 this last, as the outer part of the wire would stretch more than the 

 inner would compress. It may be taken as *046 inch at the place 

 chosen, which was unfortuuately near the top. As 1 millim. is *0394 

 inch, Mr. Broun's statement on this point must be rejected. 



I cannot imagine on what ground this plunger has been made of 

 glass. Surely it cannot be contended that a metal wire would be of 

 uneven diameter, in a way that a glass one would not be ? On the 

 other hand the risk of injury is considerable, and such a rod would be 

 impossible to replace in foreign parts. 



DESCRIPTION OF PLATES 8 AND 9. 



As lias been mentioned in the note to p. 519 the plates were prepared subsequent 

 to the submission of the Report, which they are intended to illustrate. 



Plate 8 is a general oblique view of the gravimeter on its tripod stand. The chief 

 parts seen are — the shaft through which the parallel wires descend : — the chamber 

 through the glass front of which is seen obscurely what is represented very faithfully 

 in Plate 9 : — the collimator, or observing telescope, to which a somewhat undue pro- 

 minence is given in the drawing, by the effect of foreshortening : — the table and 

 foot-screws : — the tripod stand : — the well and glycerine reservoir, with the cathe- 

 tometer on the left. 



There is hardly any part of this plate which requires more special explanation than 

 will be found in the foregoing pages, if we except perhaps the arrangement of nuts 

 and screws on the outside of the chamber ; and these will be readily understood by 

 consulting the other plate, where it is seen that they serve to govern the positions of 

 the two stages inside by which the hanging part (the major weight of the Report) is 

 held so as to maintain a fixed position, with slackened supporting wires, when 



2 Q 2 



