112 



NA TURE 



{June I, 1876 



its rate at Greenwich. The statement of the Astronomer 

 Royal in one of his annual Visitation Reports, that the 

 rate of the clock "maybe considered certain to much less 

 than one second per week," does not, we believe, over- 

 estimate its performance. As regards its absolute vari- 

 ation from true time, we find, according to his last re- 

 port, that on 83 per cent, of the days of the preceding 

 year its error was below one second. We may mention 

 that the clock has a gravity escapement, that the com- 

 pensation of the pendulum is entirely metallic and gene- 

 rally similar to that of the Greenwich Sidereal Standard 

 (described in our article on that clock), and that the first 

 blow on the bell at the hour is true clock time, it having 

 been made a condition in the construction of the clock 

 that there should be no loss of time in the first stroke. 



So far as regards the work done from one of the wires 

 passing from the Royal Observatory : on the other, ter- 

 minating at London Bridge, currents also pass from 

 Greenwich hourly, which, with the exception of that at 

 I P.M., are placed at the disposal of the South-Eastern 

 Railway Company, who in return give to the Royal Obser- 

 vatory, for two or three minutes daily at ih., communication 

 between London Bridge and Deal for the purpose of 

 dropping at the latter place a time signal ball belonging to 

 the Admiralty, placed on the old semaphore tower (part 

 of the now abolished Navy Yard). For communication 

 with Deal at ih. a clock at London Bridge (one of those 

 before spoken of as being controlled from Greenwich) 

 automatically s-vitches the Greenwich wire into com- 

 municacion with a wire on the main line of the South- 

 Eastern Railway. Other special connections are also 

 daily made at Ashford and Deal before the current can 

 pass uninterruptedly from the Observatory to the time- 

 ball ; immediately a.ter ih. the wires are restored to their 

 former positions. To ensure that the ball has fallen 

 properly at ih., an arrangement exists by which, after 

 its discharge and before it has completed its descent, it 

 makes such momentary changes of the wire connections 

 as cause a "return" signal to pass to Greenwich indi- 

 cating that it has fallen. This ball was established by 

 the Admiralty to give Greenwich time to shipping in 

 the Downs, and has been in use since the year 1855. It 

 is placed under the superintendence of the Astronomer 

 Royal, who in his annual Visitation Reports gives statis- 

 tics by which we can judge of its practical working. Ex- 

 amining these reports for the last few years we find, on 

 the averajje, that about once in two months the ball was 

 not raised on account of high wind, and that about once 

 ill six weeks, from accidental telegraphic fault, there was 

 no discharge. An erroneous drop appears to be rare, 

 happening once or so in a year. When such does occur, 

 a black flag is at once hoisted as indicatioa of mistake, 

 and the ball is then dropped at 2h. The efficient working 

 of the ball, thus distant from the Observatory, is con- 

 sidered by the Astronomer Royal to be mainly due to the 

 establishment of the return signal which immedidtely 

 makes known at Greenwich whether the ball has fallen 

 in the usual way. 



Excepting the ih. current, used as described for the 

 Deal ball, the remaining hourly currents to London 

 Bridge are distributed by Mr. C. V. Walker mainly on 

 the lines of the South-Eastern Railway. For this distri- 

 bution the clock at London Bridge, already spoken of, 

 switches at different hours different wires into connection 

 with the Greenwich wire, and so passes on the Greenwich 

 current ; at some hours it goes to the office of the British 

 Horological Institute in Clerkenwell, for the use of watch 

 and chronometer makers. 



It will be seen that the country generally is well served 

 by the system now described, but a useful extension 

 would be made by the establishment of authoritative 

 signals in favourable positions on our coasts, for the pur- 

 pose of giving to mariners the means of obtaining Green- 

 wich time and approximate sea rates for their chronome- 



ters after leaving port. One coast signal only at present 

 exists — the time-ball set up at Deal, as already de- 

 scribed. Some few years after its erection, however, it was 

 suggested that a time-signal should be also exhibited 

 every hour at some headland of the southern coast, and 

 after some discussion of localities, the Astronomer Royal 

 proposed a detailed scheme for showing such signals on 

 the Start Point. And more recently the Shipowners' 

 Association of Liverpool made inquiry as to the facilities 

 for exhibiting a similar hourly signal on the Tuskar Rock. 

 Neither of these schemes has yet been carried into 

 execution, but, excepting the question of cost, there seems 

 to be otherwise no difficulty. 



It was indicated in an early part of our article that one 

 of the objects of connecting the Greenwich Observatory 

 with the telegraphic system was the possible determina- 

 tion of differences of longitude between Greenwich and 

 other observatories by the exchange of galvanic signals, 

 and since such connection has existed, many important 

 determinations of the kind have been made. We cannot 

 here enter into any detailed description of the different plans 

 that have been from time to time employed in practically 

 carrying out such operations : it will be sufficient to say 

 that the longitudes of the principal British and of some 

 continental observatories have been thus determined. On 

 two occasions Atlantic cables have been employed for 

 fixing the positions of points in America, and more re- 

 cently (in connection with the Egyptian expedition for 

 observation of the late Transit of Venus) the .longitude of 

 Cairo has been by similar means determined. In the 

 latter operation signals were exchanged between the sub- 

 marine cable stations in Cornwall and Alexandria with 

 perfect success, through one unbroken line of submarine 

 wire. The telegraphic method of determining longitude 

 is one of the most accurate that can be employed. 



The connection of the Royal Observatory with the 

 telegraph assists scientific inquiry and even commercial 

 enterprise in various unexpected ways. Capt. Heaviside, 

 R.E., having recently been engaged with some pendulum 

 experiments at the Kew Observatory, ic only became 

 necessary to connect the telegraph line at Greenwich to 

 the Sidereal Standard for a few minutes daily, to enable 

 hiai to receive seconds signals through the Post Office 

 wires, and so refer his observations directly to the Green- 

 wich clock. Also, in the laying of Atlantic cables, an 

 accurate knowledge of Greenwich time beiii^; 01 the 

 greatest importance for the exact navigation of cable 

 ships, Greenwich time has on such occasions been daily 

 passed from the Royal Observatory through the cable 

 Itself, as it was being submerged, to the ship. 



Our object has been simply to describe the Greenwich 

 system, but we may mention tfiat the plan of telegraphing 

 time, first carried out at Greenwich -as part of the daily 

 routine, has since been adopted in other places. In Britain 

 much has been done at the observatories of Liverpool, Edin- 

 burgh, and Glasgow, for the dissemination of a knowledge 

 of accurate Greenwich tune, both by public clocks and 

 public signals, in the vicinities of those cities. A lime-gun 

 is fired daily both at Liverpool and Edinburgh by signal 

 from the observatories of tnose places. Time-signal 

 systems in connection with observatories are also in 

 operation in various of our colonies, and in places abroad. 

 In the United States of America several very extensive 

 systems have of late years been established, and it has 

 recently been proposed to regulate the clocks of Paris 

 from the Paris Observatory. 



The system of employing the ordinary telegraph service 

 of a country for the daily transmission of time in many 

 directions from a fixed observatory shows the benefit that 

 may sometimes ensue from uniting for a special object 

 the powers of two separate institutions of totally different 

 character. The astronomer must for his own particular 

 work obtain from the face of the sky that which, espe- 

 cially in our day, is also so useful to mankind, an accurate 



