XV111 



Wardhoe. Another difficulty had also presented itself ; it was found 

 that after the planet appeared to have fairly entered upon the Sun's 

 disk it was for some time connected with the Sun's limb by a black 

 ligament. Even after the planet had separated itself from the edge, 

 it did not immediately assume a circular appearance. It is clear that 

 the determination of the precise moment of contact is more difficult 

 than it was expected to be at the time of the invention of this 

 method by Halley. For these reasons the results of the transit of 

 1874 were looked forward to with the greatest interest. The observa- 

 tions were on the w T hole successful, though some perplexing diffi- 

 culties arose. At several places the black ligament was not seen, but 

 a faint thread of light of sensible width, due to the atmosphere of 

 Yen us, presented itself. This unexpected appearance introduced an 

 element of uncertainty as to the exact moment of contact. Two 

 hundred and sixteen photographs were taken, but their examination 

 did not realise all that had been hoped for from them. On the 

 return of the several expeditions, the reduction of the observa- 

 tions was proceeded with under the scrutiny of Captain Tupman. 

 The immense labour of these calculations delayed the publication of 

 the complete results until June, 1881, nearly seven years after the 

 event, though a preliminary report was made to the House of 

 Commons in 1877. 



In the ' Phil. Trans.' for 1872 there is a curious determination of 

 the magnetic state of the iron in the Britannia and Conway tubular 

 bridges. This investigation was suggested by the peculiar tremor 

 of the iron felt by the hand when a train was passing. Both Sir 

 George Airy and Mr. Stephenson expressed this by saying that the 

 metal seemed to be in a state of " molecular shiver." As all experi- 

 ments show that iron in this state of tremor is peculiarly subject to 

 the inductive action of external magnetic force, Sir George thought 

 that observations made along the axis of the tube might lead to some 

 important conclusions. One general result was that " in the axis of 

 a rectangular tube, at all points except very near the ends, the action 

 of external magnetic forces in planes normal to the axis is absolutely 

 destroyed." Some strange anomalies were observed in one tube which 

 could not be explained until it was remembered that that tube had 

 had a fall while being raised into its place. Thus the effects of an 

 accident were discovered a quarter of a century after it had occurred 

 by a magnetic experiment. 



The system of time signals by which Greenwich time is spread over 

 the country by means of the electric telegraph is in great part due 

 to Sir G. Airy. It should be noticed that the whole system is auto- 

 matic ; the Greenwich clock being once set each morning to the exact 

 time, the signals are distributed without any person having to touch 

 the apparatus. 



