446 Rpyal Astronoinical Society. 



The Astronomer Royal then stated that the possible advantages 

 of this method appeared so great that he had begun to contemplate 

 the practicability of adopting it in the Royal Observatory. One 

 reason exists there which jjrobably exists nowhere else, namely, the 

 regularity of use of the Altitude and Azimuth Instrument by the 

 method of transits, and the necessity of referring these transits (with 

 the smallest possible uncertainty on personal equation) to the same 

 clock to which the meridional transits are referred. In adoj)ting 

 Professor Mitchell's general method, he would propose to record the 

 observations upon a cylinder, perhaps revolving upon a screw-axis. 

 It is proper to remark, that this screw-axis may be so mounted upon 

 friction-wheels that the friction would be quite imperceptible. The 

 friction- wheels must not have their ])lanes transverse to the axis, but 

 in such a position that their edges follow the threads of the screw ; 

 and, supposing that there are two friction-wheels at each end, one 

 of the four must be fixed in place, and the others must be mounted 

 on frames which have a small hinge-motion in the direction perpen- 

 dicular to the planes of the wheels. Or, instead of using the screw- 

 motion, the frame which supports the axis of the cylinder may run 

 upon a railway, along which it will be carried by rack-and-pinion 

 mechanism, receiving its movement from the clock-work. The mo- 

 tion of the cylinder may be given by a toothed wheel on one end, 

 which works in a stiff pinion, long enough to admit the toothed 

 wheel to slide along it through a space equal to the length of the 

 cylinder. 



In using this cylindrical record, it is obvious that great conve- 

 nience would be gained if the movement of the cylinder could be 

 made so perfectly uniform that it could be adopted as the transit- 

 clock. Then the second-registers could be made on it by the same 

 clock which moves it (either by galvanic contact or mechanically), 

 and their places would bear a constant relation to the lines parallel 

 to the axis of the cylinder. The Astronomer Royal, therefore, urged 

 strongly the importance of improvements of the centrifugal or coni- 

 cal-pendulum clock, as the only instrument yet made which is able 

 to do heavy work with smooth motion, and with an accuracy at 

 present so great as to make it probable that, with due modification, 

 the greatest accuracy may be obtained. Setting aside the consi- 

 deration of Fraunhofer's clock, as an instrument which, for pur- 

 poses like these, is rude, the Astronomer Royal stated that, as he 

 believed, the first efficient conical-pendulum clock was that made by 

 Mr. Sheepshanks, in which the expansion of the balls to a certain 

 angle produces suddenly a friction which, as soon as it amounts to 

 an equivalent to the maintaining power, prevents further acceleration. 

 Theoretically, the speed of this clock depends in a very small degree 

 on the maintaining power. In order to remedy this small defect, 

 the Astronomer Royal had introduced the use of w'ater power (by 

 the modern form of Barker's mill cr reaction engine) as the moving 

 force, and had regulated the supply of water by Sieman's chronome- 

 tric governor. The principle of this governor may be used in various 

 forms ; the following, perhaps, will serve to explain it most clearly. 



