VIII. FORCE. 107 



90 by rotating the lever through ,360, as at the normal station. (On pro- 

 ceeding 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 arc 

 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 millimetre 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 turning the 

 lever so as to diminish the friction, there is an additional apparatus to ensure 

 that vertical friction has no effect on the observation at last. The lever 

 contains a magnet ; and two bar magnets, with rack-work adjustments for 

 height, are placed one on each side of the instrument, so that by a pinion and 

 rack movement they can be approached to the lever magnet till their force is 

 exactly equal to the torsion force of the single wire, and the agate points are 

 no longer in contact. 



The instrument is made to serve for latitudes differing about 10 or 15, 

 but an auxiliary apparatus carries five platinum rings, which can be lowered 

 upon the weight, so as to make the instrument serve from the equator to the 

 poles, and to any height in the atmosphere. 



There are special appliances for portability, by one of which the weight is 

 fixed ; another fixes the lever ; so that strain is removed from the suspension 

 wires> and the suspended pails cannot be shaken from their places. Levels, 

 a thermometer, and other details fit the instrument for the most accurate 

 observations. The suspension wires are fixed at their ends in a special 

 manner, so that the fixed points cannot vary. All the suspended apparatus is 

 electro-gilt. 



42 Id. Photograph of Automatic Bathometer. 



A. Gerard, Lie ye* 



