289 
from the nature of the disturbing force ; it was also felt long 
before the sound of the train was audible.* I also found that 
when an object was bisected, and the level of the circle read, 
both were found to have varied after the passage of a few 
trains; and that the wires were ill-defined, and the stars blur- 
red by the transit ofa heavy engine. 
*¢ Sir James South, also, made reflection observations on 
stars, with a very powerful telescope, near Watford, on chalk 
full of fissures and very heterogenous, with results analogous 
to the preceding, but, I believe, of less intensity ; these, un- 
fortunately, have not yet been published. 
“These observations, though fully significant, were made 
under conditions much less favourable than those which exist 
in fixed observatories; but I regret that I am able to complete 
them in this respect by my own experience. In 1848 the 
Ulster Railway was opened, and I soon found reason to con- 
gratulate myself on haying resisted the original scheme which 
would have carried its line only 480 feet from my transit. The 
Armagh Observatory stands on ground probably correspond- 
ing in its vibrative power to that at Kensal Green. Under- 
neath it is about 130 feet of dense clay, resting on mountain 
limestone, and the same clay is exposed in some deep cut- 
_ tings on the line. The hill descends 90 feet, and rises from 
the valley 40 to the Terminus, which is the nearest point of 
the line, its horizontal distance being 2100 feet. The trains 
are light and few, five up and five down, and the velocity mo- 
derate, not averaging, so near the Terminus, 20 miles per hour. 
The Terminus bears 50° west of my north meridian mark, and 
* This is adverse to an opinion expressed by Mr. Robert Stephenson, that 
this tremor of mercury is chiefly caused by the sonorous vibrations of the air. 
On one day the artillery was practising with shot from 24-pounders at the 
Pigeon House, and though the reports were heavy, the mercury was compa- 
ratively little affected. 
} The average weight of engine and carriages is 40 ton, to which may be 
added 5 more for passengers. 
7 
