528 



Major J. Herschel. 



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 ea ch 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 in- 

 strument 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 parts can- 

 not 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 cuds in a special manner, 

 so that the fixed points cannot vary. All the suspended apparatus 

 is electro-gilt." 



(1.) We learn from this that the suspending wires are of gold, and 

 that suspended parts are electro-gilt. I observe that there is a 

 tendency to spottiness, resembling mould, on some parts of the gilded 

 surface. 



(2.) The part which I have called the "minor weight" is here 

 designated as a "cylindrical lever-." The latter term is but remotely 

 descriptive. 



(3.) The torsion of the double wires is not alluded to in describing 

 the forces which balance each other. This confirms my doubt 

 whether ike very important part played by this torsion was recog- 

 nised. It is an essential feature, without which the position of 

 maximum gravity- action could not be chosen j and in making the 

 adjustments it is impossible to disregard it. 



(4.) There is no necessity for the two side mirrors on the major 

 weight to be inclined at " exactly 90° " to the middle one. Indeed, it 

 is scarcely possible to tell exactly what their inclination is. It is about 

 90° ; and that is all that can be said or desired. 



(5.) Of the two terms " telescope " and " collimator," the latter 

 describes more correctly the function of the appliance by which the 

 angular positions of the mirrors are observed. The scale is in the 

 focus of the object-glass of this collimator, rather than in that of its 

 eye-piece, though, of course, the latter is also true. The divisions and 

 value of this scale are not mentioned. If my estimate is right, which 



