G3G bepoiu.' — 1884. 



opposes the induced, continuing the process until the reading of the instrument is 

 unaltered on reversing the bar. 



The equation for equilibrium is approximately 



M E cosec (6— 0) = G (I— K) 



where M is the magnetic moment of the magnet, E the total force of the earth's 

 magnetism, 6 the angle of dip, I the scale readiDg corresponding to 6, K the scale 

 reading (near the centre of the scale) in the position of no torsion, the value of 

 8 corresponding to K, and G is a constant depending on the arc value of each scale 

 division, on the distance between the suspension threads, and on the magnetism 

 induced in the bars by the suspended magnet. The value of G may be determined 

 from the time of vibration about the position of equilibrium. 



11. On an Electric Control for an Equatorial Cloclc-movement. 

 By the Earl of Rosse, F.E.S. 



Although in the construction of timekeepers with intermittent motion a 

 marvellous degree of accuracy has been arrived at, when it has been sought to 

 obtain that constant and uniform motion, so important in the driving apparatus of an 

 equatorial telescope, greatly increased difficulties have arisen. 



In all the various examples we have the approximate control effected by the 

 ordinary ball governor, whose balls, by moving outwards from the axis of rotation 

 under the action of centrifugal force, when the required velocity has been nearly 

 attained, bring into action a force of friction which prevents that velocity from 

 being sensibly exceeded even with very considerable additions to the driving power. 

 Now, although this simple contrivance has for some kinds of work proved 

 sufficiently satisfactory, yet in all its forms and modifications, whether with 

 approximate parabolic motion of the balls through a considerable range, or with 

 their motion confined within the narrowest limits between a crutch or rest and a 

 friction-ring, it has failed to afford unaided that accuracy of motion so desirable in 

 celestial photography and spectroscopy. 1 



Hence the plan has been resorted to of adjusting the governor for a speed 

 slightly too great, and reducing that speed to just what is required by means of a 

 supplementary friction-break. 



The friction-break has received several modifications. A brush dipping into 

 oil and a Qj revolving in air have been employed, and the pressure of a lever-arm 

 clothed with leather against a metal disc revolving at a high velocity, being now 

 the most usual and probably the most effective arrangement. I am now using a 

 leather covering on the disc. 



We have in all these modifications this principle in common for bringing the 

 supplementary break into play. The break is connected with a cog-wheel, rolling 

 between two others, the first of which, rigidly connected with the train and gover- 

 nor, revolves continuously at a normal speed of, say, a little greater than once a 

 minute ; the second, carrying a scape-wheel of, say, 30 teeth, is permitted by a 

 seconds pendulum to rotate intermittently in the opposite direction once in a 

 minute. The intermediate wheel thus tends to preserve its normal position, any 

 slight excess or defect of velocity of the governor bringing the friction break into 

 play for a portion of each second slightly longer, or slightly shorter, than the 

 normal one, and so preventing further displacement. 



(A). The arrangement in its simplest form is provided with an escapement and 

 a pendulum, whose motion is kept up by the action of gravity upon the intermediate 

 wheel and fittings through that escapement, but as the impelling force is augmented 

 or diminished according as the break comes more or less into action, isochronism 

 of the pendulum is only very imperfectly attained. 



(B). Hence a modification was introduced by Mr. Howard Grubb, in which the 

 pallets are inclined at the angle of friction throughout their length, and the scape- 

 wheel drops without giving an impulse to the pendulum, and the motion is kept up, 



1 Dr. Huggins in working with his spectroscope, and Mr. Common in photography, 

 have been obliged to aid this governor by continual personal at tention. 



