1863.] 631 



transit- instrument was mounted, with all the attention to stability 

 that could be given in a first-class Observatory ; and it had sufficient 

 azimuthal motion to enable the observer to follow the Pole-star in 

 its whole course ; so that night or day (if clear), he could have the 

 reflected image of the star in the mercurial vessel, ready to testify 

 against the tremors caused by any train. 



The distance of the vessel from the nearest part of the tunnel was 

 302 yards, that proposed for Greenwich being 286 yards. The length 

 of the tunnel is 1812 yards; its southern or London end is 643 

 yards from where the mercury was placed, its northern or Tring 

 end 1281 yards ; and about 64 feet of chalk and gravel lie above the 

 brickwork of its crown. The author's preparations were not com- 

 plete till December 1846, and then a continuance of cloudy weather 

 interfered with observation till January the 1 1th, 1847> when and on 

 the following nights he obtained results so decisive that he felt it 

 his duty to communicate them at once to the then First Lord of the 

 Admiralty, the late Lord Auckland, who was so satisfied with them, 

 that in a letter to Sir James, dated "Admiralty, Jan. 26, 1847," he 

 recorded the impression they had made on his mind in the following 

 words : " They would be quite conclusive if the question of carrying 

 a tunnel through Greenwich Park were again to be agitated" Sir 

 James, however, continued the work to the end of March. 



With the ordinary disturbance to which an Observatory is liable 

 (as wind, carriages, or persons moving near it), the reflected image 

 of a star breaks up into a line of stars, perpendicular to the longest 

 side of the mercury-vessel. With increased agitation, another line 

 of stars perpendicular to the first appears, making a cross. With 

 still more the cross becomes a series of parallel lines of stars ; still 

 more makes the images oscillate ; and at last all becomes a confused 

 mass of nebulous light. The first of these (the line) is not injurious 

 to onec lass of observations ; but the others are, and therefore the 

 second (the cross) was taken as a measure of the beginning and end 

 of injurious disturbance. Signal shots were fired when a train passed 

 the southern entrance of the tunnel, and a shaft 1162 yards from it. 

 Hence the train's velocity was obtained, and thence its position at 

 any given time. 



Upwards of 230 observations are given in detail, and their most 

 important results are shown in a Table, which contains the date, the 



2 Y2 



