XVII 



4 The value thus obtained is larger than that given by the Schehallien 

 method, and considerably larger than that deduced by Baily from 

 the torsion rod experiments. After the experiments at Harton Pit 

 were concluded, he gave a lecture at the Central Hall, South Shields, 

 on the pendulum experiments, and, in the next year, a Friday Evening 

 Lecture at the Royal Institution, in London, on the same subject. 

 These experiments were also noticed by " Mr. Punch " in a copy of 

 verses on " Airy and the Coal Hole," written by Shirley Brooks. 



In 1867 Professor Airy read a paper at the Institution of Civil 

 Engineers on the use of the suspension bridge with stiffened roadway 

 for railway and other bridges of large span. This paper was the result 

 of a discussion with Mr. R. Stephenson on the method to be adopted 

 for the Britannia bridge, and the author thought there were better 

 methods available for wide crossings than simple tubular bridges. 

 Besides writing this paper, for which he received the medal of the 

 Society, he often attended the meetings and joined in the discussions. 



As the management of the expeditions to observe the transits of 

 Venus in 1874 and 1882 would necessarily fall on the Astronomer 

 Royal, Mr. Airy began his preparations as early as 1857. In that 

 year, he called the attention of the Astronomical Society to the 

 means available for correcting the measure of the Sun's distance in 

 the next twenty-five years. On this occasion he pointed out the 

 peculiar advantages of observing Mars for that purpose, showing 

 that at the opposition of 1860 that planet would make a near 

 approach to the Earth. The preparations for the transit were con- 

 tinued during the following years ; the proper places to which the 

 expeditions should be sent were discussed at great length and finally 

 chosen. The northern stations having been occupied by Continental 

 observers, the southern ones were, by the advice of the Board of 

 Visitors, divided amongst the English parties. It was also decided 

 that photographic should be combined with eye observations, and an 

 extra grant was obtained from the Treasury for that purpose. By 

 1873 preparations had so far advanced that an efficient body of 

 observers from all classes, naval, military, and civilian, were collected 

 at the Royal Observatory, and were being instructed and practised in 

 all the practical details of observation with the transit, the altazimuth, 

 the equatoreal, and especially with the working model of the transit. 

 At this time the received measure of the Sun's distance depended on 

 the transits of 1761 and 1769, but mainly on the latter. Though 

 there was a close accordance in the results obtained from the different 

 transits, yet all investigators expressed doubts on their correctness. 

 In the transit of 1761 the results depended almost entirely on an 

 accurate knowledge of the differences of the longitudes of very dis- 

 tant stations. In that of 1769 the result rested in great measure 

 on the observations of a single person, viz., those of Eather Hell at 



c 



