On Gravimeters. 



527 



" 2nd. By removing a small portion of the weight, equal to that 

 due to the increased attraction of the earth, the weight can be 

 turned through exactly 90° by rotating the lever through 360°, 

 as at the normal station. (On proceeding south weight has to be 

 added.) 



" The following are the instrumental arrangements in order 

 to make these observations : — 



" The weight has on each of three sides, at its base, a vertical 

 mirror (silvered, not quicksilvered) ; the middle mirror makes 

 an angle of exactly 90° with the other two. The lever also 

 carries a vertical mirror, which, when there is no torsion in the 

 suspension wire is immediately below and in the same vertical 

 plane with the middle mirror of the weight. A telescope, having 

 a glass scale at the focus of the eye-piece, is adjusted so that 

 images of the scale can be seen (one higher than the other) 

 reflected from the middle mirror of the weight and the lever 

 mirror. When both of these mirrors are exactly in the same 

 plane, the middle division on the scale seen directly with the 

 eye-piece, coincides with the same division in the two reflected 

 images. 



"By a wheel and pinion (with endless screw and clamp for 

 delicate movement) placed below the instrument, a polished agate 

 point can be made to act on a similar agate point fixed to the 

 lever, so as to turn the latter through any angle. When turned 

 through 360° the middle scale division again agrees with the 

 image from the lever mirror. If the image reflected from one of 

 the side mirrors of the weight does not agree also, the lever is 

 turned through a greater (or lesser) angle than 360°, till this 

 agreement is obtained ; the difference of the angle through which 

 the lever has been turned from 360° is obtained from the scale 

 reading, as seen on the lever mirror. 



" The following apparatus is employed for very small increases 

 or diminutions of the weight. Suspended to and vertically below 

 the lever is a carefully calibrated glass wire (1 millim. diameter), 

 which enters a glass tube fixed below the instrument. At the 

 lower end of this tube is a cistern containing a liquid (distilled 

 water, or as at present, chemically pure glycerine). This liquid 

 can be forced into the glass tube by a screw and piston (as in 

 some barometer cisterns) . The liquid is then raised till such a 

 diminution of weight is produced by the immersion of the glass 

 wire as to bring the mirror of the weight through exactly 90°, 

 when the lever is turned through 360°. The length of glass 

 wire immersed is read, by a micrometer microscope and scale, 

 to a thousandth of a millimeter. 



" Though finely polished agate points have been employed for 



