TRANSACTIONS OF SECTION A. 637 



and the isockronism with a regulator clock is preserved by alternate equal and 

 opposite electric currents at every second, as in the ordinary system of controlled 

 clocks. 



This method appears to have proved satisfactory in the cases where it has been 

 applied, but as the 3-foot reflector at Paisonstown, being unprovided with friction 

 rollers, does not move quite so freely, and, being unprotected by a building, is 

 necessarily more affected by the wind, and as the oil employed to lubricate the parts 

 is more exposed to the action of the atmosphere, it appeared doubtful if the plan 

 would be as good in practice as it is in theory. 



For the sake of simplicity the penduluni-control without the intervention of 

 electricity, described in (A), had first been tried, but though a marked improvement 

 in the going over what it had been under the control of the governor alone was 

 observed, still there remained much to be desired. 



The ordinary electric control from a clock in the Observatory was then applied 

 to the same pendulum, tbe impulse being given through the escapement as before, 

 but the action of the control was not found sufficiently prompt. The pendulum 

 was then discarded and the escapement re-arranged, so that the pallets were per- 

 pendicular to the motion of the scape-wheel, and cut square. Thus no impulse 

 was given to it by the latter, and it was made to oscillate with the pendulum of the 

 regulator-clock by the alternate action of two electro-magnets upon two pieces 

 of soft iron attached to the escapement. The going of the clock-movement was 

 now quite satisfactory, the electric control being sharp and prompt ; but tbe battery 

 power required to work it with certainty was considerable, and though this might 

 have been much reduced by more refinement of workmanship, through diminution 

 of the amplitude of motion of the escapement, and consequent reduction of the 

 distance through which the magnets would have to exert their attractive force, it 

 appeared preferable to vary the construction. 



(C). The pallets are now inclined by an amount rather greater than the angle 

 of friction, so that under the pressure of the scape-wheel the escapement would 

 oscillate freely. Its oscillations are checked by the currents from the controlling 

 clock, which are sent alternately through each of two electro-magnets. Thus the 

 oscillations of the escapement are arrested on the attached piece of soft iron coming 

 into contact with it. 



It is now possible to lengthen the arm carrying the keeper to almost any desired 

 extent, so as to gain power, as the attractive force is no longer required to attract 

 the keeper from the opposite end of the range of motion, but simply to hold it when 

 brought into contact until the current be diverted, aud less than one-twelfth of the 

 battery power is adequate to work the modified escapement. I am unable to say, 

 cceteris paribus, the relative strengths of current required by this and by Mr. 

 Grubb's arrangement, but it may be claimed that the former (C) is by no means 

 so complicated, and that at every second it is brought under a quick and certain 

 control fiom the Observatory clock, that no refined workmanship is needed — an 

 important quality in the case of a telescope unprotected by a building, in a moist 

 climate — and that in theory the batteiy power may be reduced to any desired 

 extent by lengthening the lever-arm which carries the keeper. 



It has been found of advantage to add a ' dash-pot ' to retard the descent of the 

 scape-wheel tooth from one pallet to the next, also to place the centre of motion so 

 that the escapement may always have time to complete by its momentum its full 

 amplitude of oscillation, and come into contact with the electro-magnet even when 

 not pushed all the way by the pallet, and it may not even be necessary, in order to 

 prevent tripping, that the current be sent into each magnet as soon as, or before, it 

 ceases in the other. Probably the total duration of the currents need not be 

 greater than in arrangement (B). 



12. On Polishing the Specula of Reflecting Telescopes. 

 By the Earl op Rosse, F.R.S. 



So little has been written on the polishing of specula, so few, comparatively, 

 have had experience in the operation and the process ia considered to be attended 



