282 THE SEA. 



they both listen to the ticking of the clock marking the seconds, and separately calculate the 

 results of their observations, which are afterwards compared. To obtain a greater degree of 

 certitude, they occasionally exchange places. In this way the slightest chances of error are 

 eliminated. The aberrations of the instrument must also be taken into account. Notwith- 

 standing its excellence and the solidity with which it is fixed to stone walls sunk into the 

 ground, it sometimes is affected by slight vibrations, which can only be attributed to the terra 

 finna on which it is constructed. Mr. Airy has noticed this same phenomenon at Cambridge, 

 whence he has come to the conclusion ' that the surface of the earth, commonly regarded as 

 the base of all solidity, is itself in movement/ 



"'I am going to show you the clock which sets the time for all England,' said 

 the Astronomer Royal to me, as he conducted me into a little room occupying one 

 of the oldest parts of the edifice. Covered with its simple mahogany case, this Mother 

 clock, as it is called, is not unlike one of those venerable wooden-cased clocks that 

 one meets with sometimes in the old English manor-houses. No one, however, could 

 fail to discover that the mechanism in this time-keeper is new and uncommon. Its 

 chief characteristic is that it possesses two distinct attributes. In the first place it 

 marks the time most exactly; and, in the next, it communicates this power to other 

 clocks as well. It has therefore been called the Mother clock, because it animates in 

 the Observatory eight of its daughters. Its dial is divided into three circles, one of 

 which marks the hours, another the minutes, and a third the seconds. One hand 

 only moves round each of these dials, and thus points out the generally-accepted 

 measures of time. 



"The Observatory transmits signals every hour to the telegraph-office in Lothbury, 

 in the City of London, whence, by a network of galvanic wires, the knowledge of 

 the true time is spread along the lines of railway to the extremities of Great Britain. 

 This vast ^Eolian harp covers thus with its chords nearly the whole surface of the 

 British Isles, and vibrates in unison with one prime mover. 



"As regards the true time, these telegraphic wires have a double mission. The 

 current leaving Greenwich transmits the signal given by the clock at the Observatory, 

 and what is called a return current then communicates the errors of the other clock 

 on which the Mother has just acted. 'I would never undertake to regulate a clock 

 from which I did not get regular replies/ said the Astronomer Royal; and just as 

 we were passing in front of a galvanic apparatus, ' Stop ! ' he added, 'the great 

 clock at Westminster is at this very moment giving me an account of itself; it goes 

 well, and is only the twentieth part of a second slow. Twice a day in this way it 

 keeps me informed of the state of its health/ '' 



Below Greenwich one of the saddest catastrophes of the century occurred in 1879, 

 one which has its lessons for all who voyage. We refer to the loss of the Princess- 

 Alice. A pleasure steamer, one of the largest and best known of the London, 

 Steam-packet Company, with some 700 happy merrymakers, a large proportion of whom 

 were children, left London Bridge on Tuesday morning, September 3rd, 1878, for 

 Gravesend and Sheerness, and everything, even the temper of our uncertain climate, 



