50 



NATURE 



[May 1 8, 1876 



may recur at any time, and, as is most probable, very 

 suddenly ; we can only hope that observers will be equal 

 to the next occasion. 



The Minor Planets. — Of the members of this group, 

 in addition to the four older ones, Ceres, Pallas, Juno, 

 and Vesta, at present favourably placed for observation, 

 the brighter are Hera, Iris, and Melpomene ; Hera and 

 Melpomene are a little below the tenth magnitude, and 

 Iris about 9*5. The following are approximate positions 

 for Greenwich midnight : — 



TH£ GREENWICH TIME SIGNAL SYSTEM 



TN Nature for April i of last year (vol. xi. p. 431) we 

 ■*■ gave a description of the new Sidereal Standard Clock 

 of the Royal Observatory at Greenwich. Fundamentally 

 important as is this clock in all that concerns its relation 

 to exact astronomical science, it performs also another 

 and more immediately practical duty, that of regulating 

 the time of great part of the United Kingdom. And we 

 propose now to trace the connection existing between this 

 purely astronomical clock and those by which the daily 

 business of our lives is arranged. 



A few words of preliminary history may not be un- 

 interesting. Formerly, when, comparatively speaking, 

 little communication existed between the people of 

 different towns, each place kept its own local time. But 

 when railways began to be extended through the country 

 in all directions, such manner of reckoning time could not 

 with any regard to convenience be followed in arranging 

 the movements of trains. The adoption of one uniform 

 system of counting time having, as regards railways, thus 

 become a necessity, all towns in connection with railways, 

 as a matter of convenience, fell sooner or later into the 

 same system, one now universally followed. The time of 

 the meridian of Greenwich is that employed. This selec- 

 tion was probably in part accidental. The railway autho- 

 rities, when seeking for uniformity, would naturally be led 

 to take as standard the time of the most influential place, 

 and so adopt metropolitan time, which happens to be, 

 practically, Greenwich time. But however this may be, 

 the selection was for another reason a happy one. The 

 meridian of Greenwich is that from which longitudes are 

 counted on all British maps, and Greenwich time having 

 been already long used by the navigator, means of ob- 

 taining a proper knowledge of it at seaports was very 

 desirable. Its adoption for railways by facilitating the 

 after-introduction of the time-signal system as now existing 

 was therefore a fortunate circumstance. 



The regular exhibition of accurate time for public 

 use, by any kind of authoritative signal, was commenced 

 at Greenwich in the year 1833, when the first time- 

 ball was erected on the eastern turret of the ancient 

 portion of the Observatory buildings, principally for 

 the purpose of giving Greenwich time to chronometer 

 makers and seamen. It has been dropped every day 

 since the year mentioned, excepting only during some 

 periods of repair, and occasionally on days of violent wind. 

 The ball, which is about five feet in diameter and 

 painted black, is by mechanical means raised half-way up 

 its mast at 5 min. before ih. as a preparatory signal ; at 

 3 min. before ih. it is hoisted to the summit. It drops at 

 ih. true Greenwich mean solar time. Formerly it was 

 discharged by an attendant who, watching a clock the" 

 error of which had been previously ascertained, pressed 

 the ball-trigger at the proper instant, but since the year 

 1852 it has been discharged by automatic means, as will 

 be explained further on. The first start of the ball, or its 



separation from the cross (indicating the cardinal points) 

 immediately above, is very sudden, and is the phase to 

 be noted ; afterwards (to avoid injury to the building), 

 a piston, connected by a long rod to the ball, falls into a 

 nearly air-tight cylinder, and so checks its descent that 

 it comes gently to rest at the foot of the mast. 



Within a few years of the establishment of the Green- 

 wich ball, others were erected at British observatories 

 near to ports and harbours, as Edinburgh, Liverpool, 

 Glasgow, &c., principally also for the service of shipping. 

 And such signal balls or equivalent means of exhibiting 

 time are now to be found at many observatories abroad, 

 as for instance at the Cape of Good Hope, Madras, 

 Bombay, Sydney, Melbourne, Mauritius, Quebec, Wash- 

 ington, Sac. Originally such time-balls could only be 

 dropped at an observatory or institution at which time 

 was determined by celestial observation, but on the intro- 

 duction of the electric telegraph an observatory could be 

 made the centre of a system from which, by galvanic 

 means, time-balls coald be dropped at, or time-signals 

 given to, distant points. 



On the first establishment of the electric telegraph in 

 England, the connection of the Royal Observatory with 

 the telegraphic system and its possible application to the 

 daily distribution of time throughout the kingdom soon 

 engaged the attention of the Astronomer Royal, but 

 before things had come to any definite shape, the scheme 

 for laying a submarine cable between England and 

 France was proposed, and active steps taken to carry it 

 out. The progress of this work was watched with interest 

 by astronomers on both sides of the channel, and some of 

 the active members of the Institute of France having 

 expressed their earnest desire to take advantage of the 

 new cable for galvanic determination of the difference of 

 longitude between the Observatories of Paris and Green- 

 wich, the Astronomer Royal became enabled in the year 

 1852, principally with the assistance of Messrs. E. Clark 

 (of the then existing Electric Telegraph Company) and 

 C. V, Walker (of the South-Eastern Railway Company), 

 to establish the long-desired communications on the 

 English side. The application of the telegraph to the 

 direct determination of longitude will not, however, 

 further concern us at present. As soon as telegraphic 

 connection with the Royal Observatory was complete, 

 the system of transmitting time signals from Greenwich 

 for distribution by the Electric Telegraph Company on 

 their lines was commenced, special apparatus having 

 been for the purpose prepared both at Greenwich and 

 London, This we now proceed to describe. 



The Mean Solar Standard Clock of the Royal Observa- 

 tory, the principal clock of the whole time-signal system, 

 erected in the year 1852 specially for the work, is always 

 kept adjusted as nearly as possible to exact Greenwich 

 mean time. It is a clock of Shepherd's construction, 

 with seconds pendulum, and is maintained in action by 

 galvanic means alone. But it works others sympatheti- 

 cally. The wire which carries the galvanic currents from 

 the pendulum to the electro-magnets to drive the hands 

 is continued, before returning to the battery, to other 

 electro-magnets in connection with the hands of other 

 dials in different parts of the Observatory building, so 

 that the hands on all the dials advance simultaneously, 

 the forward motion of the whole system depending entirely 

 on the one pendulum of the standard clock. Of these 

 various clocks, one is fixed in the boundary-wall of the 

 Observatory ; it is daily consulted by great numbers of 

 people, and will be familiar to every visitor to Greenwich 

 Park, Several are placed in the Chronometer Room for 

 use in the daily comparison of the Royal Navy and other 

 chronometers, the difference between the time shown on 

 one of these dials and that of any chronometer giving 

 immediately the error of the chronometer without further 

 calculation. Other dials are to be found in different office 

 rooms in which accurate time is necessary. All these 



